Update plugin org.springframework.boot to v2.5.6

Author: renovate-bot

build.gradle

@@ -9,7 +9,7 @@ plugins {
     id 'pmd'
     id 'checkstyle'
     id 'com.github.spotbugs' version '5.0.0-beta.1'
-    id 'org.springframework.boot' version '2.5.5'
+    id 'org.springframework.boot' version '2.5.6'
     id 'io.spring.dependency-management' version '1.0.11.RELEASE'
     id 'org.jetbrains.gradle.plugin.idea-ext' version '1.1'
     id 'com.google.cloud.tools.jib' version '3.1.4'