doc/fingerprints.txt: Add the OpenSSL OMC PGP key fingerprint

doc/fingerprints.txt: Add the OpenSSL OMC PGP key fingerprint

We want to move to using this key for tarball and announcement signatures.
It won't happen immediately, though, as we must have it specified in the
latest update of each release branch, so people can verify properly.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21015)