Unify maven-shade-plugin version in parent POM

Closes gh-14440

Author: philwebb

spring-boot-project/spring-boot-parent/pom.xml

@@ -206,7 +206,7 @@
 			<dependency>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-shade-plugin</artifactId>
-				<version>2.2</version>
+				<version>${maven-shade-plugin.version}</version>
 			</dependency>
 			<dependency>
 				<groupId>org.apache.maven.plugin-tools</groupId>