pom: remove source/target configuration on maven-compiler-plugin

On Java 11, which we require for building, the compiler accepts
--release 8 and no longer requires -source or -target, so these are
unnecessary

http-service/pom.xml

@@ -167,6 +167,8 @@
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-compiler-plugin</artifactId>
 				<configuration>
+					<source>1.8</source> <!-- required for mapstruct -->
+					<target>1.8</target> <!-- required for mapstruct -->
 					<annotationProcessorPaths>
 						<path>
 							<groupId>org.mapstruct</groupId>