Pre-BigSur, MacOS updates were labeled as "minor" releases, so we had
to bake separate builds for different 10.X releases.
In BigSur era, it doesn't seem to be the case, so for now we can re-use
our BigSur builds across all BigSur versions (11.0, 11.1 and 11.2).
If we ever need to have a custom build for some bigsur minor version,
e.g. `11.6`, we'll have a new browser platform along with generic
`mac11` platform.
Fixes#4775.
ff20b9d107