Update pipelineBranchDefaultsProjectFactory
As part of upgrading Jenkins, some Job DSL code must be upgraded.
Removed GIMP 2.8 from the list of versions of GIMP to build. After consulting with #gimp
on irc.gimp.org I learned it is no longer necessary to run CI for GIMP 2.8 since it is now unsupported.
Relates to GNOME/gimp#3378 (closed)