Change priority extra to priority optional.

Fixes lintian: priority-extra-is-replaced-by-priority-optional
See https://lintian.debian.org/tags/priority-extra-is-replaced-by-priority-optional.html for more details.
1 job for master in 2 minutes and 27 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #64327
test_the_archive

00:02:27