proj: prepare 0.12.1 release #1410

Job	Run time
Valgrind	54s
Build+test (clang, stable, ubuntu-latest)	56s
Build+test (clang, beta, ubuntu-latest)	51s
Clippy	18s
ensure-header-updated	1m 3s
Build+test (clang, nightly, ubuntu-latest)	58s
Check for documentation errors	17s
Build+test (clang, 1.61.0, ubuntu-latest)	1m 38s
Format	13s
Check minimum versions	38s
Build+test (gcc, stable, ubuntu-latest)	50s
Build+test (gcc, beta, ubuntu-latest)	51s
Windows	2m 13s
Build+test (gcc, nightly, ubuntu-latest)	1m 5s
Windows CMake, Debug configuration	2m 4s
Windows CMake, Release configuration	2m 26s
Build+test (gcc, 1.61.0, ubuntu-latest)	1m 33s
Build+test (macos-latest, clang, stable)	1m 11s
cargo-c	36s
Clang Tidy	13s
Clippy nightly (optional)	20s
Miri	54s
	22m 2s

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