Track removal of cephadm adoption compatibility pin

## Context

PR #825 pins the cephadm adoption fixture to `quay.io/ceph/ceph:v19.2.5`. The bundled MicroCeph Ceph 20.2.1 client cannot authenticate with the AES256KRB5 CephX key format emitted by Ceph 19.2.6.

## Why this needs tracking

CVE-2025-30156 is fixed in Ceph 19.2.6 and 20.2.4. The temporary 19.2.5 fixture pin preserves compatibility with the current MicroCeph snap, but keeps the fixture on the pre-fix Ceph release.

- CVE: https://docs.ceph.com/en/latest/security/CVE-2025-30156/
- Upstream compatibility change: https://github.com/ceph/ceph/commit/13063f42c2add051a4c4ac812597390eaa29c74c

## Exit criteria

1. The MicroCeph snap bundles Ceph 20.2.4 or later.
2. The cephadm adoption fixture removes the 19.2.5 pin and exercises a 19.2.6-or-later producer image.
3. The adoption test proves the newer CephX key format works without exposing the credential in CI logs.

Read PR #825 and inspect the cephadm adoption fixture and test it changed; the issue does not name their file paths. First confirm the MicroCeph snap bundles Ceph 20.2.4 or later, then check that the fixture uses a 19.2.6-or-later producer and the adoption test verifies the newer CephX key format without logging credentials.

Written by the indexing model from the issue text.