Use PL/Java 1.6.0 to save more tedium.

PL/Java 1.6.0's SQL generator is able to order UDTs and functions
it generates so that manually-added provides/requires are not needed
to make sure a UDT is created before functions that refer to it as
parameter or return type. By bumping the pljava-api dependency to
1.6.0, many of the annotations tediously added in commit d32f84e
for earlier PL/Java versions can now be avoided.

With PL/Java 1.6.0, the minimum supported Java release is 9, so edit
the POM to use a newer maven-compiler-plugin and specify 9 as the
Java release.

Author: jcflack

java/pom.xml

@@ -9,10 +9,9 @@
 		<plugins>
 			<plugin>
 				<artifactId>maven-compiler-plugin</artifactId>
-				<version>2.4</version>
+				<version>3.8.1</version>
 				<configuration>
-					<source>1.6</source>
-					<target>1.6</target>
+					<release>9</release>
 				</configuration>
 			</plugin>
 			<plugin>
@@ -38,7 +37,7 @@
 		<dependency>
 			<groupId>org.postgresql</groupId>
 			<artifactId>pljava-api</artifactId>
-			<version>1.5.1</version>
+			<version>1.6.0</version>
 		</dependency>
 		<dependency>
 			<groupId>junit</groupId>