Also bump Protobuf to latest version

This avoids an incompatibility with the latest version of Abseil, fixed here:
https://github.com/protocolbuffers/protobuf/commit/2364551c

Not sure why that problem isn't showing up in CI for this project, though.

PiperOrigin-RevId: 771122506

MODULE.bazel

diff --git a/MODULE.bazel b/MODULE.bazel
index 27bf0c8..8aa515e 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -46,7 +46,7 @@
 # required for testing.
 bazel_dep(
     name = "protobuf",
-    version = "30.2",
+    version = "31.1",
 )
 # TODO(lszekeres): Make this a dev dependency, as the protobuf library is only
 # required for testing.