Cleanup and align code after the Python3 transition

With v3.211130 we have a single CRABClient code base which works with all CMSSW versions, either with python2 or python3 interpreter.
We need to cleanup things.

- [x] make the crab-dev variant print a message discouraging users from using it #5134
- [x] remove old branches
- [x] take proper care of https://github.com/dmwm/CRABClient/issues/5004
- [x] update documentation, see also #5124
- [x] move also crab-pre to this version (in January?) and finally remove WMCore from CRABClient builds
- [x] Stop distributing py2 version of DBS Client inside CRAB, given now we have a pip-installable py3 client. There is no need to run py2 DBSClient inside CRABClient and users who need it in their scripts should switch to new client anyhow. #5182
- [x] the only reference to DBS Client in CRABClient is https://github.com/dmwm/CRABClient/blob/b6102286e0f166f8a433778c5c05590f5a3cf731/src/python/CRABClient/UserUtilities.py#L144 which is offered as a utility for users but not used in the client code. #5175
- [x] review code and look for other places where cleanup is needed
- [ ] and finally, improve code quality and robustness in py2/3 compatibility : #5139

@ddaina feel free to have a look and add to the list thing which I forgot. Of course cleanup is sort of a never ending work, we don't need to be perfect here, simply take care of things which will be tricky once we have forgot details, but can always do more cleanup as we hit thing.