Inherit maven-compiler-plugin configuration from openrs2-parent

This switches the release to 12, which fixes a javadoc issue when
building with -source 8 on JDK 12:

https://bugs.openjdk.java.net/browse/JDK-8212233

pom.xml

@@ -80,14 +80,6 @@
     <sourceDirectory>${project.basedir}/src</sourceDirectory>
     <testSourceDirectory>${project.basedir}/test</testSourceDirectory>
     <plugins>
-      <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
-        <artifactId>maven-compiler-plugin</artifactId>
-        <configuration>
-          <source>1.8</source>
-          <target>1.8</target>
-        </configuration>
-      </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-surefire-plugin</artifactId>