When Rocq 9.0 is our minimum...

[jdchristensen]

Sometime over the next few months, I recommend that we make Rocq 9.0 (released in March 2025) our minimum supported version. When we decide to do so, here are some tasks:

• Update various places that indicate what the minimum version is, e.g. coq-hott.opam.template, flake.nix, maybe more.
• Update CI to no longer do 8.x builds.
• Update README.md and INSTALL.md to mention Rocq instead of Coq.
• Update library name from Coq-HoTT to Rocq-HoTT (or HoTT-Rocq if we want a funny name).
• Update the build scripts to use the new binary name, e.g. rocq instead of coqc. Then remove the opam dependency on coq-core.
• Handle the TODO in Basics/Overture about internal_paths_rew.
• Handle other TODO items that apply (search for 8.19, 8.20, 9.0).

The first two items should be done together and be done first, but the rest are fairly independent, so they can be done in independent PRs as people find the time.

Feel free to add tasks to the list or make edits.