Bump com.google.firebase:firebase-database from 20.3.1 to 21.0.0 (#228)

Bumps [com.google.firebase:firebase-database](https://github.com/firebase/firebase-android-sdk) from 20.3.1 to 21.0.0.
- [Changelog](https://github.com/firebase/firebase-android-sdk/blob/master/docs/make_release_notes.py)
- [Commits](https://github.com/firebase/firebase-android-sdk/commits)

---
updated-dependencies:
- dependency-name: com.google.firebase:firebase-database
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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app/build.gradle

@@ -61,7 +61,7 @@ dependencies {
     implementation 'com.firebaseui:firebase-ui-auth:8.0.2'
     implementation 'com.google.firebase:firebase-auth:23.0.0'
     implementation 'com.google.firebase:firebase-messaging:24.0.0'
-    implementation 'com.google.firebase:firebase-database:20.3.1'
+    implementation 'com.google.firebase:firebase-database:21.0.0'
     implementation 'com.google.firebase:firebase-storage:21.0.0'
     implementation 'com.google.firebase:firebase-firestore:25.0.0'
     testImplementation 'junit:junit:4.13.2'