Compile Error Prone annotations artifact as Java 8

Support for the Java 7 language level is being turned down, see: https://bugs.openjdk.org/browse/JDK-8173605

RELNOTES=The Error Prone annotations artifact now targets Java 8
PiperOrigin-RevId: 479355036

annotations/pom.xml

@@ -49,8 +49,8 @@
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-compiler-plugin</artifactId>
         <configuration>
-          <source>1.7</source>
-          <target>1.7</target>
+          <source>8</source>
+          <target>8</target>
           <compilerArgs combine.self="override" />
         </configuration>
       </plugin>