Upgrade to Spring Boot 2.3.1

[jkroepke]

Hi,

today dependabot raise our spring-boot-starter-parent to 2.3.1 which results into new errors while running the native-image compiler (invoked by maven):

 org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter was unintentionally initialized at build time. To see why org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter got initialized use -H:+TraceClassInitialization
org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter was unintentionally initialized at build time. To see why org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter got initialized use -H:+TraceClassInitialization

	at com.oracle.svm.core.util.UserError.abort(UserError.java:68)
	at com.oracle.svm.hosted.classinitialization.ConfigurableClassInitialization.checkDelayedInitialization(ConfigurableClassInitialization.java:518)
	at com.oracle.svm.hosted.classinitialization.ClassInitializationFeature.duringAnalysis(ClassInitializationFeature.java:187)
	at com.oracle.svm.hosted.NativeImageGenerator.lambda$runPointsToAnalysis$8(NativeImageGenerator.java:720)
	at com.oracle.svm.hosted.FeatureHandler.forEachFeature(FeatureHandler.java:70)
	at com.oracle.svm.hosted.NativeImageGenerator.runPointsToAnalysis(NativeImageGenerator.java:720)
	at com.oracle.svm.hosted.NativeImageGenerator.doRun(NativeImageGenerator.java:538)
	at com.oracle.svm.hosted.NativeImageGenerator.lambda$run$0(NativeImageGenerator.java:451)
	at java.util.concurrent.ForkJoinTask$AdaptedRunnableAction.exec(ForkJoinTask.java:1386)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

I'm using 0.7.0 from Milestone repo.

See for full maven log: https://github.com/adorsys/keycloak-config-cli/pull/100/checks?check_run_id=764286508