Update build to support Bazel 8/bzlmod

[bstoll]

This PR updates GNMI proto generation to be more closely tied to Bazel, similar to other repos like GNOI with the regenerate-files.sh script.

• Go toolchain is now pinned with Bazel.
• bazel/tools.go is added to pin go proto compilers into go.mod
• BUILD files are added for all packages (generated by Gazelle).
• Python proto support is removed.
• GRPC/Proto toolchains updated to latest releases.
• protos use import_prefix - this should help these issues when using reflection and paths are relative to each repo instead of globally unique: Is there a recommended way to generate descriptor files for the gnoi protos? gnoi#216 , pathz grpcurl reflection: included an unresolvable reference to ".gnmi.Path" gnsi#176 , gRIBI grpcurl: Failed to resolve symbol "gribi.gRIBI" gribi#37 , gnmi_ext proto error when doing protoreflect  #95
• Gazelle target at root will auto generate BUILD files for Go.

This PR is the Copybara output in Google from cl/707303444.