Bump rxjava from 2.2.20 to 2.2.21 (#1874)

Bumps [rxjava](https://github.com/ReactiveX/RxJava) from 2.2.20 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@v2.2.20...v2.2.21)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>
Co-authored-by: Aaron Klish <aklish@gmail.com>

elide-core/pom.xml

@@ -102,7 +102,7 @@
         <dependency>
             <groupId>io.reactivex.rxjava2</groupId>
             <artifactId>rxjava</artifactId>
-            <version>2.2.20</version>
+            <version>2.2.21</version>
         </dependency>
 
         <!-- JSR 303 Validation -->