Bump default theme versions for WordPress 6.9

[sabernhardt]

Bundled themes will need a version bump for the 6.9 release.

Previously:
#63681 - 6.8.2
#62990 - 6.8
#62034 - 6.7
#60701 - 6.6
#59816 - 6.5
#58832 - 6.4
#57857 - 6.3
#57689 - 6.2
#56450 - 6.1
#55754 - 6.0
#54783 - 5.9
#53277 - 5.8
#52704 - 5.7
#51919 - 5.6
#50889 - 5.5
#49743 - 5.4

[sabernhardt]

Except for Twenty Twenty-Three, all of the themes have at least one change (as of today).

[sabernhardt]

#63709 was marked as a duplicate.