Addons: rename fields on API response (#11764)
Instead of using `version_a` and `version_b`, we are using `current` and
`base` for these field names. These names are standard GitHub/Git names,
for example when creating pull requests and you have to select the "base
branch".
* Refs:
https://github.com/readthedocs/readthedocs.org/pull/11762#issuecomment-2471388313
* Refs:
https://github.com/readthedocs/readthedocs.org/pull/11749#discussion_r1836357613
