Update release notes for WatchListClient feature support

[wallrj]

Preview: https://deploy-preview-1572--cert-manager.netlify.app/docs/releases/release-notes/release-notes-1.16#memory-optimizations

In https://github.com/cert-manager/cert-manager/pull/7315 I realised that there's a simpler, more standard way to enable client-go features, and I've documented that here.

We cherry-picked https://github.com/cert-manager/cert-manager/pull/7315 to release-1.16

/hold

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[wallrj]

/unhold

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