Investigate using `cargo-rbmt` for MSRV dep management

[notmandatory]

Describe the enhancement

Investigate using cargo-rbmt to manage our MSRV dependencies. This tool is used by the rust-bitcoin project.

Use case

We currently maintain a ci/pin-msrv.sh file. With this tool I think we just check in two cargo.lock files. One for our latest rust version and one for our minimum.

Impact

• Blocking production usage
• Nice-to-have / UX improvement
• Developer experience / maintainability

Additional context

Suggested by @luisschwab here:
https://discord.com/channels/753336465005608961/1368973459551555605/1480926691856748585

[luisschwab]

Once we get this fleshed out here, we can do the same the rest of the repos in the org.