Release: correct build date in verification; other improvements

- the date is actually the date of the commit *prior*
  to the tag commit, as the files are built and then committed.
- also, the CDN should still be checked for non-stable releases,
  and should use different filenames (including in the map files).
- certain files should be skipped when checking the CDN.
- removed file diffing because it ended up being far too noisy,
  making it difficult to find the info I needed.
- because the build script required an addition, release
  verification will not work until the next release.
- print all files in failure case and whether each matched
- avoid npm script log in GH release notes changelog
- exclude changelog.md from release:clean command
- separate the post-release script from release-it for now, so we
  can keep manual verification before each push. The exact command is
  printed at the ened for convenience.

Closes gh-5521

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