Revert support for multiple keys in attr_encrypted

What does this MR do and why?

Support for multiple encryption keys was recently added, but in practice we still don't allow to define multiple keys in config/secrets.yml.

Since there's an MR to use the upstream attr_encrypted gem again: !186797 (merged) and that we can use Active Record Encryption (which support multiple encryption keys), we're reverting the multiple keys support in attr_encrypted here, so that we can use the upstream gem.

We should migrate all attr_encrypted fields to Active Record Encryption in the near future.

References

• Unblocks Update attr_encrypted gem (!186797 - merged)

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