Adjust pom.xml to compile in java 21

Add java 21 to the matrix in Github CI
Fixes #26

.github/workflows/maven.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ 8, 11, 17 ]
+        java: [ 8, 11, 17, 21 ]
     name: Java ${{matrix.java}}
     steps:
       - uses: actions/checkout@v2

pom.xml

@@ -195,6 +195,7 @@
 
 	<profiles>
 		<profile>
+			<id>java6</id>
 			<activation>
 				<jdk>1.6</jdk>
 			</activation>
@@ -213,6 +214,19 @@
 				<javadoc.opts>-Xdoclint:none</javadoc.opts>
 			</properties>
 		</profile>
+		<profile>
+			<id>java</id>
+			<activation>
+				<!--
+					release 1.7 is removed in java 20
+					https://bugs.openjdk.org/browse/JDK-8173605 
+				-->
+				<jdk>[20,)</jdk>
+			</activation>
+			<properties>
+				<compiler.level>1.8</compiler.level>				
+			</properties>
+		</profile>
 		<profile>
 			<id>deploy</id>
 			<build>