[1.6.x] Fixed #22454 - Changed compatibility warning hint for test runner.

Backport of 214d1e1b0f from master
[1.6.x] Fixed #22471 -- Corrected misprint in i18n docs

Backport of 54d5c37de6572eae57a66339bb38719e681cee82 from master.
[1.6.x] Added some missing items to the 1.6.3 release notes.

Backport of c2b269df6e from master
[1.6.x] Fixed #22473 -- Emphasized that Model.clean() is not called from Model.save().

Backport of b43bbd6d39 from master
[1.6.x] Fixed #22458 -- Added a note about MySQL utf8_unicode_ci collation

Thanks tobami at gmail.com for the report.

Backport of 11ac50b18e from master
[1.6.x] Moved RemoteUserBackend documentation to reference guide.

Backport of 26d118c3fe from master