Update pom.xml to support Java 8.

We last checked that it needed to support Java 7 a long time ago.  We made a change a couple
of months ago that used lambdas.  Instead of backing out that change, let's push Java 8
support and see if anyone complains.

PiperOrigin-RevId: 339091712

pom.xml

@@ -141,8 +141,8 @@
         <artifactId>maven-compiler-plugin</artifactId>
         <version>3.6.1</version>
         <configuration>
-          <source>1.7</source>
-          <target>1.7</target>
+          <source>1.8</source>
+          <target>1.8</target>
         </configuration>
       </plugin>
       <!-- configure eclipse project -->