make `qlever` the `main` branch

[egonw]

Now that scholia.toolforge.org no longer works, and while we are working on setting up the new solution on WMF platforms (see #2766), it is time to continue our work on the qlever branch from #2715

My proposal is:

1. rebase the 'qlever' branch on the latest 'main'
2. branch the current 'main' to 'main-legacy'
3. merge the rebased 'qlever' branch to 'main'