[1.7.x] Fixed #22250 -- regression in gis compiler for non-concrete fields

Thanks to gwahl@fusionbox.com for the report.

Backport of 21f208e66e from master
[1.7.x] Fixed #21858 -- Clarified 1.6 release note regarding M2M help text changes.

Thanks lee at semel.net for the report.

Backport of f30e6590f4 from master
[1.7.x] Increased memoization cache size for language codes.

There may be more than 100 (default maxsize) commonly seen xx-yy values
on some sites. The additional memory consumption isn't significant.

Also added a comment explaining why this cache must have a maxsize.

Backport of f356b6e from master.