Remove macos-11 tester from github actions.

[MFraters]

splitting up #712, because the macos-11 runner has been removed now, so there is no point asking for it anymore.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	0.991 ± 0.005 (s=455)	0.991 ± 0.005 (s=456)	-0.2% .. +0.1%
Slab interpolation curved simple none	0.996 ± 0.004 (s=434)	0.995 ± 0.004 (s=472)	-0.1% .. +0.0%
Spherical slab interpolation simple none	1.159 ± 0.006 (s=388)	1.158 ± 0.006 (s=391)	-0.1% .. +0.1%
Slab interpolation simple curved CMS	1.031 ± 0.003 (s=455)	1.031 ± 0.003 (s=420)	-0.1% .. +0.1%
Spherical slab interpolation simple CMS	1.585 ± 0.010 (s=278)	1.585 ± 0.011 (s=292)	-0.2% .. +0.2%
Spherical fault interpolation simple none	1.168 ± 0.008 (s=402)	1.168 ± 0.009 (s=371)	-0.2% .. +0.1%
Cartesian min max surface	2.380 ± 0.018 (s=194)	2.376 ± 0.020 (s=187)	-0.4% .. +0.1%
Spherical min max surface	7.021 ± 0.036 (s=69)	7.015 ± 0.037 (s=62)	-0.4% .. +0.2%