bump to v29.0.0-rc.12 #260

Job	Run time
changed-files-miri	23s
changed-files	3s
build-docs	2m 18s
test-on-windows-latest	7m 2s
test-on-ubuntu-latest	2m 29s
test-on-macos-latest	1m 52s
test-on-ubuntu-latest-arm32	1m 34s
test-on-ubuntu-latest-aarch64	1m 38s
test-on-windows-latest-aarch64	4m 2s
test-on-ubuntu-latest-use_meter left_handed	2m 28s
test-on-ubuntu-latest-lightweight async-trait	2m 32s
test-with-miri	0s
all-tests-passed	1s
auto-merge	0s
	26m 22s

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