Disable broken sphinx-version-warning
Disable broken sphinx-version-warning

The doc build is currently broken because of this dependency:
```
Exception occurred:
  File "/home/vagrant/.local/lib/python2.7/site-packages/versionwarning/extension.py", line 5, in <module>
    from .signals import generate_versionwarning_data_json
  File "/home/vagrant/.local/lib/python2.7/site-packages/versionwarning/signals.py", line 41
    **{config.versionwarning_message_placeholder: '<a href="#"></a>'},
```

Related: #6460

Signed-off-by: Thomas Graf <5f50a84c1fa3bcff146405017f36aec1a10a9e38@cilium.io>

cilium/cilium,scanf/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,scanf/cilium,cilium-team/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,scanf/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,cilium-team/cilium,michi-covalent/cilium,scanf/cilium,cilium/cilium,tklauser/cilium,scanf/cilium,scanf/cilium,scanf/cilium