fix(crd): remove spec.storage.retainOnDelete rather than leave it inert

The field validated, shipped in the CRD, and reached no controller. The world
PVC survives deletion unconditionally -- it is a StatefulSet VolumeClaimTemplate,
StatefulSet deletion does not cascade to template PVCs, and no finalizer exists
anywhere in the operator. So setting it true described what already happened,
and setting it false did nothing at all. False is the worse half: it reads as a
request to delete a world, and was silently ignored.

This departs from spec v4.1 §5, which asks for
"删除:finalizer 清 Service/STS/ConfigMap,PVC 按 retainOnDelete". Neither half
was ever built. Restoring that line means adding a finalizer whose other listed
duties -- Service, StatefulSet, ConfigMap -- ownerReference GC already performs,
so the only work it would newly do is delete worlds, on a path that does not
pass the reaper's verified-backup check. The reaper is the one thing in the
system allowed to destroy a world and it earns that by proving a backup first.
A second door without that check is not an improvement.

The spec is a frozen versioned document, so it is left alone and the departure
is recorded in troubleshooting.md §13, beside the behaviour it explains. §12
loses its inert row and its opening sentence, which existed to introduce this
one field: every field in that table is now read by a controller.

Deployed installs need nothing. A CR still carrying retainOnDelete keeps
working, because a v1 CRD prunes unknown keys on the next write and the
behaviour the field claimed to control was never conditional.

go build, go vet and go test ./... pass on Linux with zero failures; the CRD
still parses and storage keeps size and storageClassName.

deploy/crd/felis.lolicon.best_minecraftservers.yaml

@@ -269,10 +269,6 @@ spec:
               storage:
                 description: Storage configures the world PVC.
                 properties:
-                  retainOnDelete:
-                    description: RetainOnDelete keeps the PVC when the MinecraftServer
-                      is deleted.
-                    type: boolean
                   size:
                     description: Size is the requested PVC capacity (e.g. "10Gi").
                     type: string