Bump com.google.protobuf from 0.9.5 to 0.9.6 (spiffe#389)

Bumps com.google.protobuf from 0.9.5 to 0.9.6.

---
updated-dependencies:
- dependency-name: com.google.protobuf
  dependency-version: 0.9.6
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

java-spiffe-core/build.gradle

@@ -1,5 +1,5 @@
 plugins {
-    id 'com.google.protobuf' version '0.9.5'
+    id 'com.google.protobuf' version '0.9.6'
     id 'java-library'
     id 'java-test-fixtures'
 }