Mark snapshot version of protobuf gradle plugin non changeable (elastic#138241) (elastic#138256)

This should avoid Gradle to check for updates on that Snapshot artifact even though
we know it will not change. This should make CI more stable towards flaky
network issues with jitpack

Author: breskeby

x-pack/plugin/otel-data/build.gradle

@@ -12,7 +12,9 @@ buildscript {
     maven { url = "https://jitpack.io" }
   }
   dependencies {
-    classpath 'com.github.google:protobuf-gradle-plugin:PR786-SNAPSHOT'
+    classpath('com.github.google:protobuf-gradle-plugin:PR786-SNAPSHOT') {
+      changing = false
+    }
   }
 }