Rework 1.12 release notes

[SgtCoDFish]

1. Moves the main release description to the top so it's easily visible without having to scroll all the way down to v1.12.0 to see it.
2. Adds global known issues at the top of the doc so they're clearly visible
3. Calls out the dupe secretname issue in global known issues
4. Adds a note about 1.12 being LTS

Prompted by https://github.com/cert-manager/cert-manager/issues/6992

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[ThatsMrTalbot]

/lgtm

[SgtCoDFish]

/approve

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