Update dependency undici to v6.15.0 (#952)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [undici](https://undici.nodejs.org) ([source](https://togithub.com/nodejs/undici)) | [`6.14.1` -> `6.15.0`](https://renovatebot.com/diffs/npm/undici/6.14.1/6.15.0) | [![age](https://developer.mend.io/api/mc/badges/age/npm/undici/6.15.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://developer.mend.io/api/mc/badges/adoption/npm/undici/6.15.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://developer.mend.io/api/mc/badges/compatibility/npm/undici/6.14.1/6.15.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/undici/6.14.1/6.15.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>nodejs/undici (undici)</summary>

### [`v6.15.0`](https://togithub.com/nodejs/undici/releases/tag/v6.15.0)

[Compare Source](https://togithub.com/nodejs/undici/compare/v6.14.1...v6.15.0)

#### What's Changed

-   Expose EnvHttpProxyAgent to Node.js core bundle, so it can be turned … by [@&#8203;mcollina](https://togithub.com/mcollina) in [https://github.com/nodejs/undici/pull/3148](https://togithub.com/nodejs/undici/pull/3148)
-   test: add headerslist copy check by [@&#8203;tsctx](https://togithub.com/tsctx) in [https://github.com/nodejs/undici/pull/3156](https://togithub.com/nodejs/undici/pull/3156)
-   chore: ensure automated v6 release compared to v6 by [@&#8203;mweberxyz](https://togithub.com/mweberxyz) in [https://github.com/nodejs/undici/pull/3149](https://togithub.com/nodejs/undici/pull/3149)
-   fetch: do not leak signal listeners by [@&#8203;mcollina](https://togithub.com/mcollina) in [https://github.com/nodejs/undici/pull/3158](https://togithub.com/nodejs/undici/pull/3158)
-   fix: request cache mode is not the same as request mode by [@&#8203;tsibley](https://togithub.com/tsibley) in [https://github.com/nodejs/undici/pull/3151](https://togithub.com/nodejs/undici/pull/3151)
-   fetch: don't re-lowercase HeadersList by [@&#8203;tsctx](https://togithub.com/tsctx) in [https://github.com/nodejs/undici/pull/3159](https://togithub.com/nodejs/undici/pull/3159)
-   fix casing issue when cloning Headers object by [@&#8203;KhafraDev](https://togithub.com/KhafraDev) in [https://github.com/nodejs/undici/pull/3160](https://togithub.com/nodejs/undici/pull/3160)
-   build(deps): bump node from `6d0f18a` to `db8772d` in /build by [@&#8203;dependabot](https://togithub.com/dependabot) in [https://github.com/nodejs/undici/pull/3163](https://togithub.com/nodejs/undici/pull/3163)
-   fix header cloning bug by [@&#8203;tsctx](https://togithub.com/tsctx) in [https://github.com/nodejs/undici/pull/3162](https://togithub.com/nodejs/undici/pull/3162)
-   chore: change bench naming for h2 by [@&#8203;metcoder95](https://togithub.com/metcoder95) in [https://github.com/nodejs/undici/pull/3165](https://togithub.com/nodejs/undici/pull/3165)
-   expose WebSocket related events in node bundle by [@&#8203;KhafraDev](https://togithub.com/KhafraDev) in [https://github.com/nodejs/undici/pull/3167](https://togithub.com/nodejs/undici/pull/3167)
-   feat: add support for if-match on retry handler by [@&#8203;metcoder95](https://togithub.com/metcoder95) in [https://github.com/nodejs/undici/pull/3144](https://togithub.com/nodejs/undici/pull/3144)
-   fix: correct firing order of abort events by [@&#8203;tsctx](https://togithub.com/tsctx) in [https://github.com/nodejs/undici/pull/3169](https://togithub.com/nodejs/undici/pull/3169)
-   create fast MessageEvent by [@&#8203;KhafraDev](https://togithub.com/KhafraDev) in [https://github.com/nodejs/undici/pull/3170](https://togithub.com/nodejs/undici/pull/3170)
-   chore: add explicitly [@&#8203;fastify/busboy](https://togithub.com/fastify/busboy) by [@&#8203;Uzlopak](https://togithub.com/Uzlopak) in [https://github.com/nodejs/undici/pull/3172](https://togithub.com/nodejs/undici/pull/3172)
-   chore: remove sinon as dev dependency by [@&#8203;Uzlopak](https://togithub.com/Uzlopak) in [https://github.com/nodejs/undici/pull/3171](https://togithub.com/nodejs/undici/pull/3171)
-   webidl changes by [@&#8203;KhafraDev](https://togithub.com/KhafraDev) in [https://github.com/nodejs/undici/pull/3175](https://togithub.com/nodejs/undici/pull/3175)
-   preserve dictionary key name in webidl errors by [@&#8203;KhafraDev](https://togithub.com/KhafraDev) in [https://github.com/nodejs/undici/pull/3176](https://togithub.com/nodejs/undici/pull/3176)

#### New Contributors

-   [@&#8203;tsibley](https://togithub.com/tsibley) made their first contribution in [https://github.com/nodejs/undici/pull/3151](https://togithub.com/nodejs/undici/pull/3151)

**Full Changelog**: nodejs/undici@v6.14.1...v6.15.0

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packages/next/package.json

@@ -279,7 +279,7 @@
     "timers-browserify": "2.0.12",
     "tty-browserify": "0.0.1",
     "ua-parser-js": "1.0.37",
-    "undici": "6.14.1",
+    "undici": "6.15.0",
     "unistore": "3.5.2",
     "util": "0.12.5",
     "uuid": "9.0.1",