build(deps-dev): bump spring-core from 4.3.4.RELEASE to 4.3.21.RELEASE (#6)

Bumps [spring-core](https://github.com/spring-projects/spring-framework) from 4.3.4.RELEASE to 4.3.21.RELEASE.
- [Release notes](https://github.com/spring-projects/spring-framework/releases)
- [Commits](spring-projects/spring-framework@v4.3.4.RELEASE...v4.3.21.RELEASE)

---
updated-dependencies:
- dependency-name: org.springframework:spring-core
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

pom.xml

@@ -65,7 +65,7 @@
         <dependency>
             <groupId>org.springframework</groupId>
             <artifactId>spring-core</artifactId>
-            <version>4.3.4.RELEASE</version>
+            <version>4.3.21.RELEASE</version>
             <scope>test</scope>
         </dependency>