Bump spring.version in /samples/spring/pet-store (#320)

Bumps `spring.version` from 5.1.9.RELEASE to 5.2.3.RELEASE.

Updates `spring-webmvc` from 5.1.9.RELEASE to 5.2.3.RELEASE
- [Release notes](https://github.com/spring-projects/spring-framework/releases)
- [Commits](spring-projects/spring-framework@v5.1.9.RELEASE...v5.2.3.RELEASE)

Updates `spring-context` from 5.1.9.RELEASE to 5.2.3.RELEASE
- [Release notes](https://github.com/spring-projects/spring-framework/releases)
- [Commits](spring-projects/spring-framework@v5.1.9.RELEASE...v5.2.3.RELEASE)

Updates `spring-context-indexer` from 5.1.9.RELEASE to 5.2.3.RELEASE
- [Release notes](https://github.com/spring-projects/spring-framework/releases)
- [Commits](spring-projects/spring-framework@v5.1.9.RELEASE...v5.2.3.RELEASE)

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

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

Author: dependabot[bot]

samples/spring/pet-store/pom.xml

@@ -26,7 +26,7 @@
     <properties>
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
-        <spring.version>5.1.9.RELEASE</spring.version>
+        <spring.version>5.2.3.RELEASE</spring.version>
         <junit.version>4.12</junit.version>
         <log4j.version>2.8.2</log4j.version>
     </properties>