Drop java 6 support

Fixes #4220.

BUILD

@@ -608,7 +608,7 @@ java_library(
     ]) + [
         ":gen_well_known_protos_java",
     ],
-    javacopts = ["-source 6", "-target 6"],
+    javacopts = ["-source 7", "-target 7"],
     visibility = ["//visibility:public"],
 )
 
@@ -617,7 +617,7 @@ java_library(
     srcs = glob([
         "java/util/src/main/java/com/google/protobuf/util/*.java",
     ]),
-    javacopts = ["-source 6", "-target 6"],
+    javacopts = ["-source 7", "-target 7"],
     visibility = ["//visibility:public"],
     deps = [
         "protobuf_java",

java/pom.xml

@@ -96,8 +96,8 @@
           <artifactId>maven-compiler-plugin</artifactId>
           <version>3.6.0</version>
           <configuration>
-            <source>1.6</source>
-            <target>1.6</target>
+            <source>1.7</source>
+            <target>1.7</target>
           </configuration>
         </plugin>
         <plugin>