Remove macOS 11 GitHub Actions runners #119

Job	Run time
Build (ubuntu-latest, stable)	14s
Build (ubuntu-latest, 1.36.0)	1m 46s
Cargo Deny (ubuntu-latest, stable)	3m 0s
Format (ubuntu-latest, stable)	5s
Build (macos-latest, stable)	17s
Clippy (ubuntu-latest, stable)	9s
Format (ubuntu-latest, nightly)	12s
Build (windows-latest, stable)	1m 28s
Clippy (ubuntu-latest, nightly)	16s
Build (windows-latest, 1.36.0)	2m 4s
Test (ubuntu-latest, stable)	20s
Test (ubuntu-latest, 1.56.0)	1m 53s
Test (macos-latest, stable)	24s
Test (windows-latest, stable)	2m 0s
Test (windows-latest, 1.56.0)	2m 30s
	16m 38s

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