Document how to update Cargo.lock.msrv

[piodul]

When dependencies are added or updated in Cargo.toml, it's frequently the case that Cargo.lock.msrv needs to be updated, too. Failing to do so may break the MSRV cargo check with features step in the rust_min workflow with the following message:

Updating crates.io index
error: the lock file /home/runner/work/scylla-rust-driver/scylla-rust-driver/Cargo.lock needs to be updated but --locked was passed to prevent this
If you want to try to generate the lock file without accessing the network, remove the --locked flag and use --offline instead.
Error: Process completed with exit code 101.

The message doesn't explain what went wrong and how it should be fixed. We need to:

• Add an instruction to CONTRIBUTING.md on how and when to update Cargo.lock.msrv,
• Make a short instruction or a reference to CONTRIBUTING.md appear in the output of the MSRV cargo check with features step so that contributors see it when investigating the failure in CI logs.

cc: @Lorak-mmk