Add repository dispatch for EC release notes

[diamonwiggins]

Example PR that's generated when this dispatch is triggered: #3246. I believe there are no release notes as the release notes generator does a diff between the current version and the last release version.

The above PR was tested manually with a workflow dispatch on the diamonwiggins/test-workflow-dispatch branch with:

curl -X POST \
  -H "Accept: application/vnd.github+json" \
  -H "Authorization: Bearer $GITHUB_TOKEN" \
  https://api.github.com/repos/replicatedhq/replicated-docs/actions/workflows/embedded-cluster-release-notes.yml/dispatches \
  -d '{
    "ref": "diamonwiggins/test-workflow-dispatch",
    "inputs": {
      "version": "2.4.0+k8s-1.30" 
    }
  }'

https://github.com/replicatedhq/replicated-docs/blob/diamonwiggins/test-workflow-dispatch/.github/workflows/embedded-cluster-release-notes.yml

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[paigecalvert]

it looks like the slack notification part is commented out, but if you wanted to include it, just a heads up that I just bumped slackapi/slack-github-action@v2.0.0 to 2.1.0

[diamonwiggins]

Thanks Paige! Will bump that and uncomment when https://github.com/replicatedcom/ITandSecurityRequests/issues/665 is ready.