Decouple migrations: Announce backwards compatibility guarantee

Created by: efritz

Partially implements Step 0 in RFC 469.

Write an announcement for the #dev-announce channel, as well as an email to the engineering organization outlining the changes that will be made in CI going forward. The CI check to be added later should not come as a surprise to any developer that has currently planned work.

Links to the docs produced in #25250 (closed) should be included/linked.