Add workflow to update release notes from milestone

[maraf]

Release notes are currently written manually for each release, following different templates for api-* and blazor-* releases. This adds a manually triggered GitHub Action to automate that process.

How it works

The workflow takes two inputs -- a milestone title and a release tag -- and generates the release body by:

1. Detecting the release type (api or blazor) from the tag prefix
2. Looking up the milestone by title (searches both open and closed milestones)
3. Fetching all closed issues (excluding PRs) from the milestone
4. Building the release body matching existing templates:
   • blazor-*: site URLs + milestone link + issue list
   • api-*: same as above, plus a Docker containers section with a GHCR package link
5. Updating the release via gh release edit

Usage

Go to Actions > "Update Release Notes" > Run workflow, and provide:

• milestone: e.g., api-v1.9.0
• release_tag: e.g., api-v1.9.0

Notes

• The workflow requires contents: write to update releases and packages: read to look up Docker container package IDs for API releases.
• If the GHCR package lookup fails (e.g., container not yet published), it falls back to a plain text container reference without a link.