Thanks Sylvain Garancher for the report and Veranika Sabiashchanskaya for the
initial patch.

Refs #17375.

Thanks iktyrrell for the patch.

RendererMixin will soon be removed but this removal and the corresponding
test changes stand on their own.

Thanks Collin for the report.

Thanks Tim for the suggestion.

Thanks Ankush Thakur for the report and Simon for the review.

The previous version of Argon2 uses encoded hashes of the form:
   $argon2d$m=8,t=1,p=1$<salt>$<data>

The new version of Argon2 adds its version into the hash:
   $argon2d$v=19$m=8,t=1,p=1$<salt>$<data>

This lets Django handle both version properly.

Unused since afe0bb7b.

Thanks Anders Roos for the report.

The test sometimes fails on CI and isn't worth debugging at this time.

Unused since afe0bb7b.

Thanks for Simon Willison for the spot.