Exclude `UPDATE` operations in constraints for immutable fields #1277

Job	Run time
Verify assertions in suite.yaml files	22s
generate	2m 40s
Require a suite.yaml file alongside every template.yaml	10s
Require a sync.yaml file and metadata.gatekeeper.sh/requires-sync-data annotation for every templ...	2m 16s
Test scripts	2m 32s
Unit test on ubuntu-latest opa v0.42.2	13s
Unit test on macos-latest opa v0.42.2	17s
Integration test on Gatekeeper release-3.13	10m 27s
Integration test on Gatekeeper release-3.12	9m 16s
	28m 13s

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