gnupg2: update to 2.5.20

gnupg2: update to 2.5.20

New stable release on the 2.5.x development series. Highlights from
upstream's NEWS:

 * gpgsm: Implement GCM encryption.

 * gpgsm: New option --attribute and server command SETATTR to
   include arbitrary signed or unsigned attributes into a
   signature. Requires libksba >= 1.7.0 (bumped to 1.8.0 in the
   preceding commit).

 * gpgsm: Introduce system attribute _signingCertificateV2.

 * gpg: Fix wrong assertion failure which could very rarely occur
   during key signature checking.

 * gpg: Consider certify-only keys for revocation signature check.

 * gpgsm: Fix possible double free in the CMS parser.

 * gpgsm: Fix possible too early removal of ephemeral keys.

Signed-off-by: Daniel Golle <redacted>