ci: Remove support for cirrus-ci based CI

Andres Freund <andres@anarazel.de>

Commit: 68c8a365d4dcefd7ec60f7a99a7ab7b557c36357
Author: Andres Freund <andres@anarazel.de>
Date: 2026-06-04T18:55:57Z
ci: Remove support for cirrus-ci based CI

As mentioned in the earlier commit, cirrus-ci has shut down. Therefore remove
all files related to running CI via cirrus. Also update comments / code that
were referencing cirrus-ci.

Discussion: https://postgr.es/m/3ydjipcr7kbss57nvi67noplncqhesl5eyb6wgol4ccjxynspv%40yatlykpribmm

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