Prepare `2.12.1rc1`. (#16238)
Prepare `2.12.1rc1`. (#16238)

[ci skip-rust]
[ci skip-build-wheels]
benjyw/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants