Set source/target release based on Gerrit v3.4 specs

Gerrit v3.4 supports Java 8 source-compatible code and has
a default Java 11 runtime: pin the expected releases and avoid
using what the Maven's defaults are for the source/target versions.

Change-Id: I8cbac74490f94b06651dc6efc74714f5329ac849
diff --git a/pom.xml b/pom.xml
index be8b163..6d7a5ae 100644
--- a/pom.xml
+++ b/pom.xml
@@ -81,8 +81,8 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.6.1</version>
                 <configuration>
-                    <source>${java.specification.version}</source>
-                    <target>${java.specification.version}</target>
+                    <source>1.8</source>
+                    <target>11</target>
                 </configuration>
             </plugin>
             <plugin>