[Documentation] Remove notice to use the release branch of the core modules - 2.3.1 has been released
[Dependencies] Bump version requirements of the core modules to 2.3.1

PDELab doesn't work correctly with the 2.3.0 release of the core
modules, so we better reflect that in the dependency requirements stated
in the dune.module file.
[Documentation] Update release notes and bump version to 2.0.0-rc2