The CentOS 7 documentation test relies on PostgreSQL 9.5 which has been removed from the yum.p.o repository package. Switch the test to CentOS 8 to fix the immediate issue, but a decision on the PostgreSQL 9.5 documentation will need to be made before the next release.
Testing on Travis-CI has been getting slower (from ~18 minutes to 3-6 hours) and the travis-ci.org service will be terminated at the end of the year. Moving to travis-ci.com is an option but the quotas are too low for our purposes.
Instead use Github Actions, which does not currently have quotas, and runs our current tests with just a few tweaks.
This still leaves multi-architecture tests on Travis-CI but we may be able to run those and stay within the new quotas.
Also fix a minor bug in restoreTest.c exposed by Github Actions using a different name for the user and group.
