[gapic-generator] Update gprcio version to `1.83.0` in pqc showcase integration tests

[ohmayr]

Currently, our pqc showcase tests run against grpc==1.83.0rc0 to verify that pqc is supported for grpc clients. Update it to grpc==1.83.0 once it's released.

Also update the showcase_pqc nox session in gapic-generator to use grpc==1.83.0 and remove --pre.