Bump io.reactivex.rxjava2:rxjava from 2.2.13 to 2.2.21 in /app

Bumps [io.reactivex.rxjava2:rxjava](https://github.com/ReactiveX/RxJava) from 2.2.13 to 2.2.21.
- [Release notes](https://github.com/ReactiveX/RxJava/releases)
- [Changelog](https://github.com/ReactiveX/RxJava/blob/v2.2.21/CHANGES.md)
- [Commits](ReactiveX/RxJava@v2.2.13...v2.2.21)

---
updated-dependencies:
- dependency-name: io.reactivex.rxjava2:rxjava
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

app/build.gradle

@@ -56,7 +56,7 @@ dependencies {
     annotationProcessor 'androidx.lifecycle:lifecycle-compiler:2.5.1'
 
     //rxjava2
-    implementation 'io.reactivex.rxjava2:rxjava:2.2.13'
+    implementation 'io.reactivex.rxjava2:rxjava:2.2.21'
     implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'
     implementation 'com.squareup.sqlbrite3:sqlbrite:3.2.0'
     implementation 'com.jakewharton.rxbinding2:rxbinding-appcompat-v7:2.2.0'