Use less specific language about DB fix version

What does this MR do?

To dovetail with the release post fix in gitlab-com/www-gitlab-com!124035 (merged), this merge request changes the upgrade notes to be less specific about which version the database migration problem in #408304 (closed) will be fixed. We originally thought it would be 15.11.1, but that's turned out to be a security-only patch release. It's probably 15.11.2 but we don't actually know that answer yet, so let's be vague rather than continually updating the info.

People who are involved with shepherding this fix to production: @jon_jenkins, @alexives, @bprescott_.

Evan, I'm comfortable with Alex doing a post-merge review here; I'd feel a lot better if we could get this out relatively quickly.

Related issues

• Related to Failure in update_major test (#408304 - closed)
• Related to Change patch info from 15.11.1 to ??? (gitlab-com/www-gitlab-com!124035 - merged)

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