Remove EL7 test configurations

Remove all the EL7 testing. EL7 is EOL from the point of view of testing.

With this we can also move from yum to dnf everywhere so we can avoid installing '/usr/bin/yum' in EL10 by default. For info the yum package is available still as the wrapper it was in 8 and 9.

The docs and javascript in linuxops have already had 7 removed by the looks of things.

https://gitlab.cern.ch/linuxsupport/testing/image-ci/-/pipelines/12101685 To check I did the correct thing - visited https://gitlab.cern.ch/linuxsupport/testing/image-ci/-/pipelines/new changed the branch to this one and fired it. Tests look to be using dnf so assume correct.

All physical machine tests failing with "Fault message: No valid host was found. "