Pin grpcio minimum in latest grpc/python version (#2108)

gRPC's python runtime (`grpcio`) [inserts a runtime
check](https://github.com/grpc/grpc/blob/7d256a4a7796e3447854938216a23727913030f1/src/compiler/python_generator.cc#L738-L781)
to ensure that the runtime is the same version or newer than the
generated plugin code. (This is unfortunately undocumented other than in
code, hence why we haven't had this minimum included before.)

Author: stefanvanburen

plugins/grpc/python/v1.60.0/buf.plugin.yaml

@@ -16,4 +16,4 @@ registry:
     requires_python: ">=3.7"
     deps:
       # https://pypi.org/project/grpcio/
-      - "grpcio"
+      - "grpcio>=1.60.0"

plugins/grpc/python/v1.76.0/buf.plugin.yaml

@@ -16,4 +16,4 @@ registry:
     requires_python: ">=3.9"
     deps:
       # https://pypi.org/project/grpcio/
-      - "grpcio"
+      - "grpcio>=1.76.0"