Bump io.reactivex.rxjava2:rxjava from 2.2.8 to 2.2.21 (#299)

Bumps [io.reactivex.rxjava2:rxjava](https://github.com/ReactiveX/RxJava) from 2.2.8 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@v2.2.8...v2.2.21)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

gradle/libs.versions.toml

@@ -18,7 +18,7 @@ logback = "ch.qos.logback:logback-classic:1.2.10"
 mockito = "org.mockito:mockito-core:4.3.1"
 quickTheories = "org.quicktheories:quicktheories:0.25"
 reactiveStreams-tck = { module = "org.reactivestreams:reactive-streams-tck", version.ref = "reactiveStreams" }
-rxJava2 = "io.reactivex.rxjava2:rxjava:2.2.8"
+rxJava2 = "io.reactivex.rxjava2:rxjava:2.2.21"
 rxJava3 = "io.reactivex.rxjava3:rxjava:3.1.5"
 slf4j = "org.slf4j:slf4j-api:1.7.36"
 swt-windows = { module = "org.eclipse.swt:org.eclipse.swt.win32.win32.x86", version.ref = "swt" }