Remove clang 11 and 12 CI configurations

[msimberg]

Move custom options that were set for clang 11 and 12 to other CI configurations. Part of #1193. The last patch release of clang 11/12 was 12.0.1 in July 2021, more than three years ago (https://releases.llvm.org/).

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d218034)	18288	13794	75.43%
Head commit (3ea8f18)	18288 (+0)	13794 (+0)	75.43% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1284)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[aurianer]

LGTM thanks!