[3.x] Fix GHA on Plone 5.2

[ale-rt]

This removes the tests for Plone/Python versions that are obsolete and for which running the tests on GHA would require too much magic.

[davisagli]

Please:

• increase the major version
• make sure there is a changelog saying we are dropping support for Plone 5.1 and Python < 3.8
• update the trove classifiers in setup.py

[davisagli]

Oh, this is an older branch, so increasing the major version doesn't make sense. But let's do the others.

[ale-rt]

The 3.x branch (current master) can still support 5.1/Python 2.7 and I am using it with Plone 6.1/Python 3.13.
The test action is failing now on master.
This PR just tries to fix the CI for what is possible .
I would anyway not touch anything here and focus on the master branch that will start targeting Plone 6.0+ after #57

[davisagli]

Ok