CMIP7: Improve how variant_label validation is handled in UserConfig

As part of #648 I introduced a `mip_era` argument to support CMIP7 `variant_label` validation, but I think my original idea of accessing that value with `self.config["cmor_dataset"]["mip_era"] is superior.

Start at UserConfig and the CMIP7 validation path introduced by #648; inspect how self.config["cmor_dataset"]["mip_era"] is represented and used. Done means variant_label validation obtains mip_era from that configuration location rather than requiring a separate argument, with the resulting behavior checked against the available UserConfig tests.

Written by the indexing model from the issue text.