Change **/protobuf.patch to a symlink

Replaces all the `protobuf.patch` instances in every nested module with
a symlink to `protoc/0001-protobuf-19679-rm-protoc-dep.patch`.

This reduces the duplication of the identical `protobuf` patch
everywhere. I'd originally copied the file because:

- Bzlmod requires that patches reside in the same repo. It does not
  allow target labels to other repositories.

- Target labels cannot contain a relative path.

- Windows historically doesn't allow non-admin users to create symlinks.

However, Windows should allow symlinks now if permissions are set
properly, and the Bazel CI system seems to set them:

- https://stackoverflow.com/questions/5917249/git-symbolic-links-in-windows/59761201#59761201
- https://github.com/bazelbuild/continuous-integration/blob/028b90080584ad1fbc600e647a1b801337c0abf3/buildkite/startup-windows-pdssd.ps1#L56-L60

Author: mbland

dt_patches/test_dt_patches/protobuf.patch

@@ -0,0 +1 @@
+../../protoc/0001-protobuf-19679-rm-protoc-dep.patch

dt_patches/test_dt_patches/protobuf.patch

@@ -0,0 +1 @@
+../../protoc/0001-protobuf-19679-rm-protoc-dep.patch

dt_patches/test_dt_patches_user_srcjar/protobuf.patch

@@ -0,0 +1 @@
+../../protoc/0001-protobuf-19679-rm-protoc-dep.patch

dt_patches/test_dt_patches_user_srcjar/protobuf.patch

@@ -0,0 +1 @@
+../../protoc/0001-protobuf-19679-rm-protoc-dep.patch