Docs not being built due to changes in RTD repository cloning

[amercader]

CKAN version

All published in RTD

Describe the bug

RTD switched a while ago to using shallow clones when cloning the repo. We have logic in doc/conf.py that gets old tags to show if the release is a current one or not. This has started failing when trying to build the docs:

* 2.10: https://readthedocs.org/projects/ckan/builds/22841016/
* master (2.11) https://readthedocs.org/projects/ckan/builds/22812493/

Apparently the only workaround is to ask the admins to disable shallow cloning manually. I've requested this here:

readthedocs/readthedocs.org#10953

More background on the issue:

readthedocs/readthedocs.org#5031