Switch from kdesrc-build to kde-builder

It's time.

I believe kde-builder is mature enough to be the new default.

This time we should do it while keeping Provide a migration plan in mind so kdesrc-build users do not get lost with the transition. I think by the time we finish the transition sdk/kde-builder!60 (merged) will already have been merged, so we can preemptively mention it as well.

We need to make sure we don't accidentally leave traces of kdesrc-build in the core instructions like last time, as that is the same as broken docs. In the brief period of five days we had that first attempt to switch to kde-builder, we already had at least one user confused about what works with which tool.

The linked MR also means a breaking change between kdesrc-build and kde-builder, which IMO is desirable, because then the change will be completely obvious to users and they'll start breaking certain expectations (like thinking it should work the exact same way) that will ultimately make it easier to write docs for it in the long run.