build.gradle: Update Protobuf Plugin for Gradle to 0.8.10.

The current latest version is 0.8.12, but version 0.8.11
and later require Gradle 5.6 or later which will change our
build requirements and documentation.

See:
https://github.com/google/protobuf-gradle-plugin/releases/tag/v0.8.11

build.gradle

@@ -5,7 +5,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.google.protobuf:protobuf-gradle-plugin:0.8.6'
+        classpath 'com.google.protobuf:protobuf-gradle-plugin:0.8.10'
     }
 }