Replace deprecated BACKUP_STOP_CONTAINER_LABEL setting

Version v2.36.0 deprecated the BACKUP_STOP_CONTAINER_LABEL setting and renamed it BACKUP_STOP_DURING_BACKUP_LABEL which is supposed to signal that this will stop both containers and services. Migrating is done by renaming the key for your custom value:

    env:
-     BACKUP_STOP_CONTAINER_LABEL: database
+     BACKUP_STOP_DURING_BACKUP_LABEL: database

The old key will stay supported until the next major version, but logs a warning each time a backup is taken.