Add 1.26.x testing and remove 1.24.x testing #273

See https://libraries.openmodelica.org/branches/overview.html. We need to update to the latest released version.

We are currently testing 1.24.x, 1.25.x and master. We should stop testing 1.24.x and add 1.26.x to the testsuite, so we can better see regressions in the forthcoming 1.27.x