Automate MSRV fixes

Currently, the MSRV check runs `cargo msrv verify` and fails if the MSRV is not up to date. This requires the user to run `cargo msrv` locally and then update/submit a PR with the fix.

This can be automated by having the workflow run `cargo msrv --min ` on a failure, and then automatically opening/updating a PR with the new MSRV fix.

To prevent similar breakages from occurring downstream, we could also run the MSRV check in the `check-lurk-compiles` workflow.

Locate the workflow that runs `cargo msrv verify` and the `check-lurk-compiles` workflow. Read how their failures and current version are handled, then determine the repository’s expected approach for opening or updating a fix PR. Done means the MSRV fix is automated on failure and the relevant compile-check workflow also runs the MSRV check.

Written by the indexing model from the issue text.