Fix Maven compilation for JDK 9+ by overriding Java 1.5 target

Modified maven/core/pom.xml and vm/JavaAPI/pom.xml to use Maven properties for compiler source/target settings.
Added a 'modern-jdk' Maven profile activated on JDK 9+ to set compiler source/target to 1.8.
This resolves 'Source option 5 is no longer supported' errors when building with newer JDKs in the CI matrix.

Author: google-labs-jules[bot]

maven/core/pom.xml

@@ -44,13 +44,25 @@
                 <artifactId>maven-compiler-plugin</artifactId>
 
                 <configuration>
-                    <source>1.5</source>
-                    <target>1.5</target>
+                    <source>${maven.compiler.source}</source>
+                    <target>${maven.compiler.target}</target>
                 </configuration>
             </plugin>
         </plugins>
 
     </build>
+    <profiles>
+        <profile>
+            <id>modern-jdk</id>
+            <activation>
+                <jdk>[9,)</jdk>
+            </activation>
+            <properties>
+                <maven.compiler.source>1.8</maven.compiler.source>
+                <maven.compiler.target>1.8</maven.compiler.target>
+            </properties>
+        </profile>
+    </profiles>
 
 
 </project>

vm/JavaAPI/pom.xml

@@ -13,6 +13,24 @@
     <packaging>jar</packaging>
     <name>JavaAPI</name>
 
+    <properties>
+        <maven.compiler.source>1.5</maven.compiler.source>
+        <maven.compiler.target>1.5</maven.compiler.target>
+    </properties>
+
+    <profiles>
+        <profile>
+            <id>modern-jdk</id>
+            <activation>
+                <jdk>[9,)</jdk>
+            </activation>
+            <properties>
+                <maven.compiler.source>1.8</maven.compiler.source>
+                <maven.compiler.target>1.8</maven.compiler.target>
+            </properties>
+        </profile>
+    </profiles>
+
     <build>
         <sourceDirectory>src</sourceDirectory>
         <plugins>
@@ -21,8 +39,8 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.11.0</version>
                 <configuration>
-                    <source>1.5</source>
-                    <target>1.5</target>
+                    <source>${maven.compiler.source}</source>
+                    <target>${maven.compiler.target}</target>
                     <!-- Allow compiling java.* packages -->
                     <compilerArgs>
                         <arg>-Xlint:-options</arg>