Automate MSRV fixes

[samuelburnham]

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 <CURRENT_VERSION> 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.