ci: pin MSRV dependencies with cargo update and add instructions in README

[notmandatory]

Description

Update CI to pin MSRV dependencies with cargo update instead of by changing them in Cargo.toml. Also add instructions for manually pinning MSRV dependencies in README.

Notes to the reviewers

This is following the basic approach that rust-bitcoin is using to pin MSRV dependencies.

Changelog notice

None.

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] I ran cargo fmt and cargo clippy before committing

[notmandatory]

This PR needs to go in before #156 .

[notmandatory]

Too many dependencies have changed their MSRV to 1.60 so I'm going to open a new PR to do the same for bdk-cli.