fix(deps): update dependency io.reactivex.rxjava2:rxjava to v2.2.21

Author: renovate[bot]

pom.xml

@@ -91,7 +91,7 @@
     <hk2.version>2.5.0</hk2.version>
     <jax-rs.version>2.1.1</jax-rs.version>
     <jsr353-ri.version>1.1.4</jsr353-ri.version>
-    <rxjava.version>2.2.2</rxjava.version>
+    <rxjava.version>2.2.21</rxjava.version>
     <awaitility.version>4.0.3</awaitility.version>
     <apacheds.version>2.0.0-M19</apacheds.version>
     <surefire.version>3.0.0</surefire.version>