upstream: Add key for Jiří Denemark

The keyring now contains the following keys:

  pub   dsa1024 2000-05-31 [SC]
        C74415BA7C9C7F78F02E1DC34606B8A5DE95BC1F
  uid           [ unknown] Daniel Veillard (Red Hat work email) <veillard@redhat.com>
  uid           [ unknown] Daniel Veillard <Daniel.Veillard@w3.org>
  sub   elg1024 2000-05-31 [E]
  sub   rsa2048 2016-12-01 [S]
  sub   rsa2048 2016-12-01 [E]

  pub   rsa4096 2020-07-20 [SC] [expires: 2025-07-19]
        453B65310595562855471199CA68BE8010084C9C
  uid           [ unknown] Jiří Denemark <jdenemar@redhat.com>
  sub   rsa4096 2020-07-20 [E] [expires: 2025-07-19]

The former was used to sign releases up to 6.5.0, while the
latter will be used from 6.6.0 onwards.