macos-11 is no longer available on GITHUB, so switch to macos-12/maco… #412

Job	Run time
test (gcc)	3m 42s
test (gcc, --disable-xft)	3m 48s
build (gcc)	1m 38s
test (gcc, --enable-symbols)	4m 46s
build (gcc, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 41s
build (gcc, --disable-xft)	1m 37s
build (gcc, --disable-xss)	2m 0s
build (gcc, --enable-symbols)	1m 17s
build (clang)	1m 51s
build (clang, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 57s
build (clang, --disable-xft)	1m 51s
build (clang, --disable-xss)	3m 12s
build (clang, --enable-symbols)	1m 29s
	30m 49s

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