Resolve "Milestone 1: Add TokenAuthenticatable with feature flag control"

What does this MR do and why?

This MR:

• Removes the old encrypted_token and encrypted_token_iv fields for Ci::Trigger
• Adds a new token_encrypted field including a migration to fill the newly created token_encrypted field for Ci::Trigger

The problem: The old encrypted_token field is not searchable. This is, because it is encrypted for every single Ci::Trigger with a new iv.

We also have places inside the code, where we need to decrypt the token again to return it inside our api. Therefore, we cannot just use a digest, because this could not be 'decrypted' again.

We will be using add_authentication_token_field to encrypt and decrypt the token, when necessary.

We don't need a feature flag at this point since the method find_by_token uses the old token column during migration.

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Related to #554239