chore: Update the API catalog

- Update all mixin dependencies
- Use defaults for GAX and common protos
- Update manually-specified target frameworks to netstandard2.0 where feasible
- Update patch versions to next-minor-version-beta00 (as we intend to release a minor version soon)

Currently, Google.Cloud.Spanner.Data still targets netstandard2.1
instead of netstandard2.0, as there are some aspects of ADO.NET
which changed in netstandard2.1. We might decide to target both
netstandard2.0 *and* netstandard2.1 in the future.

Author: Jon Skeet