Use built-in alertmanager config if no fallback config is provided

[JohnnyQQQQ]

What this PR does

Which issue(s) this PR fixes or relates to

Fixes #2772

Checklist

• [x] Tests updated
• [x] Documentation added
• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[CLAassistant]

All committers have signed the CLA.

[replay]

The CHANGELOG has just been cut to prepare for the next Mimir release. Please rebase main and eventually move the CHANGELOG entry added / updated in this PR to the top of the CHANGELOG document. Thanks!

[JohnnyQQQQ]

There is also one in the integration folder, which is not failing correctly. I'm investigating it.

[JohnnyQQQQ]

The thing was that the mock that we are using in the test was built manually and not using the constructor, thus no fallback config was computed. I fixed that here, and now everything works as expected. The tests were failing and I rewrote them to reflect those changes and check for the default config instead of a 412 after a deletion. For the frontend tests the expected behavior is a redirect instead of a 412 after deletion, as the fallback config is used.

[pracucci]

Nice job, thanks!