Add info about Cargo.lock.msrv to Contributing.md

[Lorak-mmk]

This is so that contributors (external or internal) are able to find information about how to deal with failures in min_rust CI job.

Fixes: #912

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[Lorak-mmk]

v2:

• Addressed review comments
• Unrelated small change: removed unnecessary backtick, as it was breaking syntax highlighting