CHANGES: rename to CHANGES.md, no longer generated

- scripts/log2changes.pl was not included in release tarballs, which broke
reproducible builds

- since log2changes uses git

CHANGES: rename to CHANGES.md, no longer generated

- scripts/log2changes.pl was not included in release tarballs, which broke
reproducible builds

- since log2changes uses git to generate the contents, it makes it difficult
to generate the same contents later (it would need to be fixed)

- the CHANGES file has outlived its purpose. the main changes are in the
RELEASE-NOTES, the rest are better tracked directly using git or on GitHub

- put a fixed CHANGES.md in there instead pointing out where the info lives
now

Closes #14331

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