Missing default priviliges after migration from databases to preparedDatabases

Please, answer some short questions which should help us to understand your problem / question better?

• Which image of the operator are you using? registry.opensource.zalan.do/acid/postgres-operator:v1.8.2
• Where do you run it - cloud or metal? Kubernetes or OpenShift? [GCP k8s]
• Are you running Postgres Operator in production? [no]
• Type of issue? [Bug report]

After migration from databases to preparedDatabases we have missing default roles for <>_db_owner user. When the database is created with preparedDatabases everything looks good, but after migration, we have missing default priviliges. Example below:

#config before migration
  users:
    xxxxxx_db_owner:
      - createdb
  databases:
    xxxxxx_db: xxxxxx_db_owner
#config after migration
  preparedDatabases:
    xxxxxx_db:
      defaultUsers: true

default privileges when a database is created by preparedDatabases

 xxxxxx_db_owner_user | data   | function | xxxxxx_db_data_reader=X/xxxxxx_db_owner_user     +
                      |        |          | xxxxxx_db_data_writer=X/xxxxxx_db_owner_user
 xxxxxx_db_owner_user | data   | sequence | xxxxxx_db_data_reader=r/xxxxxx_db_owner_user     +
                      |        |          | xxxxxx_db_data_writer=wU/xxxxxx_db_owner_user
 xxxxxx_db_owner_user | data   | table    | xxxxxx_db_data_reader=r/xxxxxx_db_owner_user     +
                      |        |          | xxxxxx_db_data_writer=awd/xxxxxx_db_owner_user
 xxxxxx_db_owner_user | data   | type     | xxxxxx_db_data_reader=U/xxxxxx_db_owner_user     +
                      |        |          | xxxxxx_db_data_writer=U/xxxxxx_db_owner_user
 xxxxxx_db_owner_user |        | function | =X/xxxxxx_db_owner_user                          +
                      |        |          | xxxxxx_db_reader=X/xxxxxx_db_owner_user          +
                      |        |          | xxxxxx_db_owner_user=X/xxxxxx_db_owner_user      +
                      |        |          | xxxxxx_db_writer=X/xxxxxx_db_owner_user
 xxxxxx_db_owner_user |        | schema   | xxxxxx_db_reader=U/xxxxxx_db_owner_user          +
                      |        |          | xxxxxx_db_owner_user=UC/xxxxxx_db_owner_user     +
                      |        |          | xxxxxx_db_writer=U/xxxxxx_db_owner_user
 xxxxxx_db_owner_user |        | sequence | xxxxxx_db_reader=r/xxxxxx_db_owner_user          +
                      |        |          | xxxxxx_db_owner_user=rwU/xxxxxx_db_owner_user    +
                      |        |          | xxxxxx_db_writer=wU/xxxxxx_db_owner_user
 xxxxxx_db_owner_user |        | table    | xxxxxx_db_reader=r/xxxxxx_db_owner_user          +
                      |        |          | xxxxxx_db_owner_user=arwdDxt/xxxxxx_db_owner_user+
                      |        |          | xxxxxx_db_writer=awd/xxxxxx_db_owner_user
 xxxxxx_db_owner_user |        | type     | =U/xxxxxx_db_owner_user                          +
                      |        |          | xxxxxx_db_reader=U/xxxxxx_db_owner_user          +
                      |        |          | xxxxxx_db_owner_user=U/xxxxxx_db_owner_user      +
                      |        |          | xxxxxx_db_writer=U/xxxxxx_db_owner_user

default privileges when a database is migrated from database to preparedDatabases

 xxxxxxxx_db_owner_user | data   | function | xxxxxxxx_db_data_reader=X/xxxxxxxx_db_owner_user  +
                        |        |          | xxxxxxxx_db_data_writer=X/xxxxxxxx_db_owner_user
 xxxxxxxx_db_owner_user | data   | sequence | xxxxxxxx_db_data_reader=r/xxxxxxxx_db_owner_user  +
                        |        |          | xxxxxxxx_db_data_writer=wU/xxxxxxxx_db_owner_user
 xxxxxxxx_db_owner_user | data   | table    | xxxxxxxx_db_data_reader=r/xxxxxxxx_db_owner_user  +
                        |        |          | xxxxxxxx_db_data_writer=awd/xxxxxxxx_db_owner_user
 xxxxxxxx_db_owner_user | data   | type     | xxxxxxxx_db_data_reader=U/xxxxxxxx_db_owner_user  +
                        |        |          | xxxxxxxx_db_data_writer=U/xxxxxxxx_db_owner_user

Start with the migration guidance in docs/user.md#from-databases-to-prepareddatabases, then trace the code paths for databases and preparedDatabases on operator v1.8.2. Compare the default-privilege behavior for a newly created and migrated database, and add a regression test if the existing test structure supports it. Done means migration preserves the default privileges shown for preparedDatabases.