Update keyring file

The squid upstream project hosts its keyring at squid-cache.org/pgp.asc. The latest tarballs for squid 6.10 are no longer being signed by the (only) key in the keyring in d/u/signing-key.asc. Let's update that keyring to include the following key:

28F8 5029 FEF6 E865 "Francesco Chemolli (code signing key) kinkie@squid-cache.org"

This key is available at the upstream keyring and is signed by the previous key included in this keyring.

Note that, although a uscan --download-current does fetch the correct source tarball from the upstream project and reports a good signature from the key above, checking the current tarball in the archive (e.g., with gpgv) will report a bad signature from that key. This happens because the upstream tarball was re-packed and the checksums changed. Also note that the only differences between the "good" and the "bad" signed tarballs are file ownership which changed due to repacking it.