Split dependabot gradle plugin (#10454)

`gradle-plugin` dependabot PRs touch so many plugins, by the time testing is done for the entire PR, _one_ of them will inevitably have a CHANGELOG conflict.
Example: #10264

Split dependabot group into two: `kotlin-gradle-plugin` and `gradle-plugin` since, though spiritually they are similar, since sometimes packages will require update to `gradle` but not `kotlin-gradle-plugin`.

Author: jmagman

.github/dependabot.yml

@@ -115,9 +115,11 @@ updates:
           - "junit:junit"
           - "org.mockito:*"
           - "org.robolectric:*"
-      gradle-plugin:
+      kotlin-gradle-plugin:
         patterns:
           - "org.jetbrains.kotlin:kotlin-gradle-plugin"
+      gradle-plugin:
+        patterns:
           - "com.android.tools.build:gradle"
       androidx:
         patterns: