Merge branch 'jc/relnotes-2.55-rust-fix'

A description in the release notes for Git 2.55.0 has been
retroactively updated to clarify that Rust support is enabled by
default, but still optional, and will become mandatory in Git 3.0.

* jc/relnotes-2.55-rust-fix:
  Rust: fix description in Release Notes to 2.55