Bump spring-core from 5.3.16 to 5.3.20 in /demo/sample-greeting

Bumps [spring-core](https://github.com/spring-projects/spring-framework) from 5.3.16 to 5.3.20.
- [Release notes](https://github.com/spring-projects/spring-framework/releases)
- [Commits](spring-projects/spring-framework@v5.3.16...v5.3.20)

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

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

Author: dependabot[bot]

demo/sample-greeting/pom.xml

@@ -11,7 +11,7 @@
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-    <spring.version>5.3.19</spring.version>
+    <spring.version>5.3.20</spring.version>
     <spring.ws.version>3.1.3</spring.ws.version>
     <spring.webflow.js.version>2.4.8.RELEASE</spring.webflow.js.version>
     <spring.integration.version>5.5.11</spring.integration.version>