Separate repository for easystack files with 2025.06 and 2023.06

PRs are going to become complicated with the new EESSI version, as there will need to be two active branches in the software_layer if we use the current approach. Rather than this, we should take the opportunity to separate out the easystack files to their own repository. This is already how dev.eessi.io operates, so conceptually should not require new design.