Removed redundant entries from POM files

Polished POM files to remove redundant entries that were causing warnings in the IDE. Some notable changes are:
- Removed spring-boot.version from sample project as they inherit it from the parent
- Removed versioning for maven-jar-plugin (was 3.0) from multiple project so it relies on the managed version
- Removed java.version from all spring-cloud-function-* modules as they inherit the one from parent.

Addressed PR comments
- removed references to maven-jar-plugin

spring-cloud-function-samples/spring-cloud-function-sample-pojo/pom.xml

@@ -52,15 +52,9 @@
 
 	<build>
 		<plugins>
-			<plugin>
-				<groupId>org.apache.maven.plugins</groupId>
-				<artifactId>maven-jar-plugin</artifactId>
-				<version>3.0.0</version>
-			</plugin>
 			<plugin>
 				<groupId>org.springframework.boot</groupId>
 				<artifactId>spring-boot-maven-plugin</artifactId>
-				<version>1.5.1.RELEASE</version>
 				<dependencies>
 					<dependency>
 						<groupId>org.springframework.boot.experimental</groupId>