Merge rust-bitcoin/rust-bitcoin#6204: ci: upgrade to cargo-rbmt v0.3.0 and fix up breaking changes

41887fd39537ff9104d69e9f8fbba3d953e21c39 Fix docs warnings (Benalleng)
1eb6ece5dcb66b67191c430a7ecb9b2228ed0ea4 ci: upgrade cargo-rbmt and fix up breaking changes (Nick Johnson)

Pull request description:

  The API files are re-generated since cargo-rbmt now de-dupes the output making it a bit easier on the eyes. The toolchain workflows were updated as well to use the explicit commands to read current versions instead of relying on the eval env vars trick.
  
  Closes #6071


ACKs for top commit:
  tcharding:
    ACK 41887fd39537ff9104d69e9f8fbba3d953e21c39
  apoelstra:
    ACK 41887fd39537ff9104d69e9f8fbba3d953e21c39; successfully ran local tests


Tree-SHA512: aa156dfab9cb00585ebc4a89995dafff7523564d189f0bd69884e4eb8c526fa4bdcec9746719af176a6f2c114b69ab55775fe6624048b17d766f551d4a515691