chore: Prepare v3.15.0-beta.0 release #7443

Job	Run time
Check codegen and manifest	1m 51s
Test Gator	2m 22s
Build and Test (1.25.8)	6m 53s
Build and Test (1.26.3)	6m 28s
Lint	2m 41s
Build and Test (1.27.1)	7m 12s
Unit test	7m 3s
Build and Test (1.28.0)	6m 43s
[Generator Resource Expansion] Build and Test	7m 6s
[Helm] Build and Test (3.7.2, gatekeeper-system)	5m 27s
[Helm] Build and Test (3.7.2, custom-namespace)	5m 18s
	59m 4s

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