fix: pom.xml to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-COMGOOGLEGUAVA-5710356
- https://snyk.io/vuln/SNYK-JAVA-NETMINIDEV-3369748
- https://snyk.io/vuln/SNYK-JAVA-ORGCODEHAUSJACKSON-3038425
- https://snyk.io/vuln/SNYK-JAVA-ORGCODEHAUSJACKSON-3038427
- https://snyk.io/vuln/SNYK-JAVA-ORGCODEHAUSJACKSON-3326362
- https://snyk.io/vuln/SNYK-JAVA-ORGCODEHAUSJACKSON-534878
- https://snyk.io/vuln/SNYK-JAVA-ORGSPRINGFRAMEWORKBOOT-5564390

pom.xml

@@ -26,7 +26,7 @@
 
   <properties>
     <spring-cloud.version>Hoxton.SR8</spring-cloud.version>
-    <spring-boot.version>2.3.8.RELEASE</spring-boot.version>
+    <spring-boot.version>2.4.0.RELEASE</spring-boot.version>
     <spring-security-oauth2-core.version>5.3.2.RELEASE</spring-security-oauth2-core.version>
     <geonetwork.version>4.2.6-SNAPSHOT</geonetwork.version>
     <jetty.version>9.4.27.v20200227</jetty.version>