Bump com.google.protobuf from 0.9.3 to 0.9.4 in /java

Bumps com.google.protobuf from 0.9.3 to 0.9.4.

---
updated-dependencies:
- dependency-name: com.google.protobuf
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

java/proto/build.gradle.kts

@@ -6,7 +6,7 @@ import subzero.DependencyVersions
 
 plugins {
     id("com.squareup.subzero.java-conventions")
-    id("com.google.protobuf") version "0.9.3"
+    id("com.google.protobuf") version "0.9.4"
 }
 
 dependencies {