Fix master version in docs
The docs are versioned as 8.0.0-alpha1 yet we are not currently
publishing 8.0.0-alpha1 snapshots, instead 8.0.0 snapshots. We will only
later qualify designated builds as 8.0.0-alpha1 at which point the docs
version can be updated to reflect that. This commit updates the docs
versions to 8.0.0.
Closes #41941