Migrate schema-migrator from control-plane

[PK85]

AC

• Move code from control-plane to KEB
• Adjust imports in KEB repo that point to schema-migrator in control-plane
• Remove gophers from CODEOWNERS in control-plane
• Build the schema-migrator image on PR in KEB
• Build the schema-migrator image in the KEB release workflow
• Update the auto bump script to bump the schema-migrator image version in the release workflow + security file should contain that image too
• Make sure Provisionier will be not affected (they still need this)
• Update docs