Update stable version even when it's inactive

[stsewd]

When stable is inactive, we don't update it when new versions are created

https://github.com/rtfd/readthedocs.org/blob/3d5ad1ccda5b25ae574e14bedbbf0098c35a6d80/readthedocs/projects/models.py#L714-L714

This leads to stable have an old commit/identifier.

When stable is activated, we trigger a build but with an old commit, then stable is refreshed and a new build is triggered to stable.

This gives two builds to stable, one to an old commit and one to the real stable.

Any reason why we have this? We even have a test for this https://github.com/rtfd/readthedocs.org/blob/3d5ad1ccda5b25ae574e14bedbbf0098c35a6d80/readthedocs/rtd_tests/tests/test_sync_versions.py#L946

The only "downside" I can see is this, but not sure what would happen. I'll try to see this locally

https://github.com/rtfd/readthedocs.org/blob/3d5ad1ccda5b25ae574e14bedbbf0098c35a6d80/readthedocs/builds/models.py#L209-L210

[humitos]

If "update stable even when inactive" means to update the identifier, I think we should do it since it doesn't affect anything and will make it be consistent with the real stable once activated.