Switch to fallback strategy in runners token encryption

The following discussion from !23412 (merged) should be addressed:

• check on production if we managed to migrate all records
• check on production if we can safely remove columns
• check in sentry if there are no exceptions related to new encryption mechanisms
• check how often we had to use fallback-plaintext strategy
• add plain-text columns to ignored columns
• remove columns in a migration following our docs
• done.