Archive changelogs prior to 13.0.0

CHANGELOG.md was getting very large, so this commit:

1. Creates changelogs/archive-$x.md for each major version of GitLab, starting at GitLab 10.
2. Archives the matching EE changelogs in that directory too.
3. Documents this in changelogs/archive.md.

Before this change:

$ du -sh {CHANGELOG*,changelogs/archive*}
312K	CHANGELOG-EE.md
908K	CHANGELOG.md
256K	changelogs/archive.md

After this change:

$ du -sh {CHANGELOG*,changelogs/archive*}
 68K	CHANGELOG-EE.md
176K	CHANGELOG.md
 44K	changelogs/archive-10-ee.md
132K	changelogs/archive-10.md
 72K	changelogs/archive-11-ee.md
208K	changelogs/archive-11.md
 76K	changelogs/archive-12-ee.md
268K	changelogs/archive-12.md
 56K	changelogs/archive-ee.md
388K	changelogs/archive.md

Closes #18526 (closed).