Graduate external.metrics.k8s.io to stable

[rexagod]

• One-line PR description: "external.metrics.k8s.io" has been in beta longer than we'd hoped it to be, but this gradutes it to stable as it's been used extensively in production environments in the ecosystem.

• Issue link: Graduate external.metrics.k8s.io to stable #5271

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@rexagod: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-enhancements-test	21e3173	link	true	/test pull-enhancements-test
pull-enhancements-verify	21e3173	link	true	/test pull-enhancements-verify

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