Disallow `/` in key names #2724

Job	Run time
build-nginx-all-arches (x86_64)	1m 21s
build-all-examples-amd64 (ubuntu-latest)	2m 1s
build-date-epoch	1m 22s
build-nginx-all-arches (aarch64)	1m 16s
source-date-epoch	1m 24s
build-all-examples-amd64 (macos-latest)	2m 3s
annotations	1m 5s
	10m 32s

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