Fix object purge during Thanos migration

Version Control Information

Source branch: sstrk/fix-thanos-migration
Target branch: devel

Commits:

* Fix object purge during Thanos migration

The thanos-query ServiceMonitor has the same name for Thanos v1 and
Thanos v2. Previously we deleted it everytime the thanos_v2 role
has been executed. However, helm does not recreate it subsequently.
We also can't easily differentiate if the ServiceMonitor has been
created by Thanos v1 or Thanos v2 as the k8s Ansible module does not
filter for labels if a name for an object is presented.
Therefore, this commit introduces an additional migration guard task
which checks if the migration already has happened. This also has the
advantage that the deletion tasks do not get executed every time.

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docs/_releasenotes/+.fixed.thanos-migration

+Fix Thanos v1 cleanup tasks during migration to prevent accidental double deletion of resources