This update for postgresql-pgagent provides the following fixes:

- Do not build postgresql11 and postgresql12 flavors where they're
not available.
- Drop postgresql94 flavor: pgsql 9.4 is EOL.
- adding postgresql12 to _multibuild (bsc#1174996)
- Remove postgresql93 from multibuild and add postgresql11.
- Fix build with PostgreSQL 11.

This update was imported from the SUSE:SLE-15-SP1:Update update project.