Support for rules_proto 6.x

[aira-xite]

In the latest version of rules_proto several breaking changes have been made, in particular rules_proto_toolchains has been moved and it breaks build of rules_rust.

ERROR: Traceback (most recent call last):
        File "..../external/rules_rust/proto/protobuf/transitive_repositories.bzl", line 4, column 75, in <toplevel>
                load("@rules_proto//proto:repositories.bzl", "rules_proto_dependencies", "rules_proto_toolchains")
Error: file '@rules_proto//proto:repositories.bzl' does not contain symbol 'rules_proto_toolchains'

Please consider adding support for it.