Upgrade Spring Boot Version for Sample App

There are some CVEs, that are fixed by the version upgrade.

Author: KarstenSchnitter

sample-spring-boot/pom.xml

@@ -18,7 +18,7 @@
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 		<maven.compiler.source>11</maven.compiler.source>
 		<maven.compiler.target>11</maven.compiler.target>
-		<spring.boot.version>2.3.4.RELEASE</spring.boot.version>
+		<spring.boot.version>2.3.12.RELEASE</spring.boot.version>
 		<keytool.plugin.version>1.5</keytool.plugin.version>
 		<keystore.token.store_password>0bzhBRNUXBR5
 		</keystore.token.store_password>