Fix grand-central hostname corruption when cluster name recurs in external DNS

[tomach]

Summary of changes

The grand-central hostname derivation used an unbounded str.replace() to insert the .gc label, so a cluster name that recurs later in its external DNS (e.g. us-west in region us-west-2) got rewritten everywhere, producing a broken hostname. It now replaces only the first (leading-label) occurrence, which is safe because external_dns is always <cluster_name>.<region>.<domain>.

Checklist

• Link to issue this PR refers to: https://github.com/crate/cloud/issues/3065
• Relevant changes are reflected in CHANGES.rst
• Added or changed code is covered by tests
• Documentation has been updated if necessary
• Changed code does not contain any breaking changes (or this is a major version change)

[plaharanne]

👍