Make GettingStartedRust.md say that we support rust stable
In #24765 (moved), we made CodingStandardsRust.md say that we support rust stable, but we left GettingStartedRust.md saying that we support rust nightly.
In #24765 (moved), we made CodingStandardsRust.md say that we support rust stable, but we left GettingStartedRust.md saying that we support rust nightly.