Update to 1.3.7f
    
    - New upstream release 1.3.7f
      - mod_tls module unexpectedly allowed TLS handshake after authentication in
        some configurations (GH#1533)
      - Unable to verify signed data: signature type 'rsa-sha2-512' does not match
        publickey algorithm 'ssh-rsa' (Bug #4491)
    - Update mod_vroot to 0.9.11
      - Addresses a bad interaction with mod_auth_file, and failed login attempts,
        which can lead to inexplicably "stuck" processes that cannot be terminated
        (https://github.com/proftpd/proftpd/issues/1384)
    - Use SPDX-format license tag