Bump rxjava from 2.2.11 to 2.2.21

Bumps [rxjava](https://github.com/ReactiveX/RxJava) from 2.2.11 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](https://github.com/ReactiveX/RxJava/compare/v2.2.11...v2.2.21)

Signed-off-by: dependabot[bot] <support@github.com>

app/build.gradle

@@ -195,7 +195,7 @@ dependencies {
     implementation 'androidx.multidex:multidex:2.0.1'
 
     implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'
-    implementation "io.reactivex.rxjava2:rxjava:2.2.11"
+    implementation "io.reactivex.rxjava2:rxjava:2.2.21"
 
     implementation 'com.bluelinelabs:conductor:2.1.5'
     implementation 'com.bluelinelabs:conductor-support:2.1.5'