[rt] Add new signing key for Daniel Wagner

Some new RT series are signed by Daniel Wagner, so we need to add Daniel's gpg key to the rt-signing-key.pgp keyring and backport the change to the stable series branches where rt releases are done signed by Daniel. This is at least down to 6.12.y as the recent 6.12.43-rt12 was signed by him.

Link: https://lore.kernel.org/lkml/175580420905.334958.3029444147013610727@nitrogen/