Bump gson from 2.8.6 to 2.9.1

Bumps [gson](https://github.com/google/gson) from 2.8.6 to 2.9.1.
- [Release notes](https://github.com/google/gson/releases)
- [Changelog](https://github.com/google/gson/blob/master/CHANGELOG.md)
- [Commits](google/gson@gson-parent-2.8.6...gson-parent-2.9.1)

---
updated-dependencies:
- dependency-name: com.google.code.gson:gson
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

processor/build.gradle

@@ -39,7 +39,7 @@ dependencies {
     api "org.ow2.asm:asm:${asmVersion}"
     api "org.ow2.asm:asm-commons:${asmVersion}"
     api "com.google.guava:guava:$guavaJREVersion"
-    api "com.google.code.gson:gson:2.8.6"
+    api "com.google.code.gson:gson:2.9.1"
     implementation 'com.google.auto:auto-common:1.1.2'
 
     def toolsJar = Jvm.current().getToolsJar()