compat: drop support for Mercurial <6.6 and Dulwich<0.21.6

This mostly corresponds to the versions included in the upcoming Ubuntu LTS; it actually has 6.7. We retain 6.6 to give a bit more leeway and since there are currently no compatibility hacks for it.

There's not really any Official Support Policy or anything for hg-git, but I generally apply bug fixes to the oldest applicable release, and then port it forwards… So nothing since 1.0 has “left” support so far, and 1.0 was from April 2022. Compared to maintaining compatibility, I'd much rather maintain multiple releases. Bug fixing is relatively easier than new feature development, and much less likely to rely on Mercurial versions.

The only “official” policy is latest Ubuntu LTS, as added in c88e8a88c5f6 back in 2012.