build(deps): bump reactor.version from 3.4.19 to 3.4.21 (#1688)

Bumps `reactor.version` from 3.4.19 to 3.4.21.

Updates `reactor-core` from 3.4.19 to 3.4.21
- [Release notes](https://github.com/reactor/reactor-core/releases)
- [Commits](https://github.com/reactor/reactor-core/compare/v3.4.19...v3.4.21)

Updates `reactor-test` from 3.4.19 to 3.4.21
- [Release notes](https://github.com/reactor/reactor-core/releases)
- [Commits](https://github.com/reactor/reactor-core/compare/v3.4.19...v3.4.21)

---
updated-dependencies:
- dependency-name: io.projectreactor:reactor-core
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: io.projectreactor:reactor-test
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

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

reactive/pom.xml

@@ -28,7 +28,7 @@
 
   <properties>
     <main.basedir>${project.basedir}/..</main.basedir>
-    <reactor.version>3.4.19</reactor.version>
+    <reactor.version>3.4.21</reactor.version>
     <reactive.streams.version>1.0.4</reactive.streams.version>
     <reactivex.version>2.2.21</reactivex.version>
   </properties>