AES-256-GCM cipher mode requires a key that is exactly 32 bytes long.
We already handle the case when the key is too long, by truncating, but
the key can also be too short in some installations. Switching to a key
that is always exactly the right length (by virtue of right-padding
ASCII 0 characters) allows encryption to proceed, without breaking
backward compatibility.

When the key is too short, encryption fails with an `ArgumentError`,
causing the web hooks functionality to be unusable. As a result, zero
rows can exist with values encrypted with the too-short key.

When the key is too long, it is silently truncated. In this case, the
key is unchanged, so values encrypted with the new too-long key will
still be successfully decrypted.