Bump spring-core from 4.3.14.RELEASE to 4.3.21.RELEASE

Bumps [spring-core](https://github.com/spring-projects/spring-framework) from 4.3.14.RELEASE to 4.3.21.RELEASE.
- [Release notes](https://github.com/spring-projects/spring-framework/releases)
- [Commits](spring-projects/spring-framework@v4.3.14.RELEASE...v4.3.21.RELEASE)

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

Author: dependabot[bot]

pom.xml

@@ -146,7 +146,7 @@
         <dependency>
             <groupId>org.springframework</groupId>
             <artifactId>spring-core</artifactId>
-            <version>4.3.14.RELEASE</version>
+            <version>4.3.21.RELEASE</version>
         </dependency>
         <dependency>
             <groupId>com.squareup.okhttp3</groupId>