chore: update lint-staged to v15, cspell to v8, bump sass to latest

[hbuchel]

Description of changes:

• updates lint-staged to latest v15 (we were on 14). When we originally implemented linting, we could not update to the latest because we had not updated to newer version of node but now we have. Verified that linting runs on staged files when making a commit
• updates cspell to latest v8 (we were on v6). Verified by making some mispelled words.
• bumps sass to latest (we were just a couple minor versions behind), verified output on staging site https://updatelintstaged.d2bfwhpcsj9awv.amplifyapp.com/

Related GitHub issue #, if available:

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Which product(s) are affected by this PR (if applicable)?

• amplify-cli
• amplify-ui
• amplify-studio
• amplify-hosting
• amplify-libraries

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• Does this PR include filetypes other than markdown or images? Please add or update unit tests accordingly.

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• Are all links in MDX files using the MDX link syntax rather than HTML link syntax?
  
  ref: MDX: [link](https://docs.amplify.aws/)
  HTML: <a href="https://docs.amplify.aws/">link</a>

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