8.12 Release Notes

[benironside]

Fixes #4380 and fixes #4379 by creating release notes.

Preview: 8.12 release notes

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[nastasha-solomon]

Just doing a quick editorial round before opening the PR up for the technical review.

[joepeeples]

@nastasha-solomon I noticed a couple of syntax details while I was working on known issues (edits below).

I also added my own known issue in a09b68c, and will have dev & product review; it could also use a look-over from you or @benironside to make sure I got the format right. Thanks!

[yctercero]

UPDATE: Created issue - elastic/kibana#175043

I messaged on slack, but just a heads up that there is another 8.12 known issue. I have yet to create a GitHub ticket for it so that I can link to it for you, but will do so first thing in the morning. I imagine the text will be something like:

Issue: Action frequency filters in rule creation and edit flows are hidden from UI. Confirmed that rules configured with these filters are running as intended.
Workaround: Use the update rule API to make changes to the action frequency filters or export the rule, update and re-import to make changes.

cc @peluja1012 @jamesspi

[approksiu]

Small change requested, otherwise great!

[approksiu]

LGTM