Remove stale experimental flags mentions #4587

Job	Run time
check feature flags (ubuntu-latest / stable)	21m 16s
check examples (ubuntu-latest / stable)	40s
check docs (ubuntu-latest / stable)	1m 39s
check format (ubuntu-latest / nightly)	15s
check licenses (ubuntu-latest / stable)	24s
check linter (ubuntu-latest / stable)	1m 37s
check minimal versions of transitive dependencies (ubuntu-latest / stable)	1m 26s
check minimal supported rust version (ubuntu-latest / msrv)	42s
check spelling (ubuntu-latest / nightly)	3m 55s
test (ubuntu-latest / stable)	10m 18s
test (macos-latest / stable)	7m 48s
test (windows-latest / stable)	10m 22s
	1h 0m 22s

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