bazel: Migrate to bzlmod for dependency management

[minor-fixes]

This change aims to fix the bazel build postsubmit workflow in a somewhat heavy-handed way: by migrating to use bzlmod to manage dependencies, and by updating said dependencies (and the supported version of bazel itself) to the newest version(s) feasible.

This involves:

• Removing the old dependency fetching infra from WORKSPACE.bazel, and adding equivalent dependency fetches to MODULE.bazel. Each version used is the newest available on BCR, with the exception of:
  • rules_proto, which encounters a Starlark error with the latest version of 7.0.2
  • protobuf, which must match versions with the toolchain loaded by toolchains_protoc; the latest version of toolchains_protoc does not have a Starlark mapping for protobuf v29.0 yet.
• Changing rules used to generate C++ protobuf/gRPC bindings from cc_grpc_library (provided by gRPC) to cpp_grpc_library (provided by rules_proto_grpc_cpp), which seems to be the current canonical ruleset for this. These rules generate proto, grpc, and mock bindings by default, so the corresponding attributes are dropped since they're no longer necessary.
• Dropping gnmi_deps.bzl, as it is no longer used to fetch dependencies, and can only confuse downstream users that also load this file.

Additionally, there are some quality-of-life changes:

• .bazelversion is added to pin the version of bazel used when invoked via bazelisk
• .gitignore is added to avoid committing bazel-generated symlinks
• Go protobuf generation is added for all protos

Tested: bazel build //... works on my system (Linux, amd64)

---

There are good reasons not to take this change, including:

• C++ proto rule changes causing downstream breakage, as some targets are removed, and the semantics of remaining targets may have changed in an incompatible way
• Downstream bazel repositories will need to migrate to bzlmod (at least partially) in order to pick up updates following this change

...and this change author has little context on the presence/severity of these issues - guidance here would be welcome!

[minor-fixes]

startblock:
    add reviewer @marcushines ;)