release: stack bundles do not pin the cassandra-migrations image version

deploy/stacks/self-managed/global.yaml.gotmpl (the cassandra.migrations.image block) only emits a tag: when an operator supplies one through values. Otherwise the tag floats to the packaged chart's default.

This means the bundle does not determine which schema migrations actually run in a customer's cluster, so "which migrations does this stack version ship" cannot be answered precisely from the bundle. Any upgrade-policy check has to fall back to "what landed in migrations/cassandra/ since the last stack tag" as a proxy.

migrations/cassandra is already a released subproject with its own tags (migrations/cassandra/vX.Y.Z), so an exact pin is available — it just isn't used.