Downgrade to Spring Boot 2.2.8 (#3343)

Some issues appeared with Spring Boot 2.3.x, so need to temporary downgrade o 2.2.8

Ref https://github.com/Activiti/Activiti/issues/3342

pom.xml

@@ -221,7 +221,7 @@
     <maven-site-plugin.version>3.9.0</maven-site-plugin.version>
     <maven-source-plugin.version>3.2.1</maven-source-plugin.version>
     <maven-surefire-plugin.version>2.22.2</maven-surefire-plugin.version>
-    <spring-boot.version>2.3.1.RELEASE</spring-boot.version>
+    <spring-boot.version>2.2.8.RELEASE</spring-boot.version>
     <versions-maven-plugin.version>2.7</versions-maven-plugin.version>
 
     <!-- configuration properties for tests -->