Use standard header for generated Go files (#4961)

As recommended by https://golang.org/pkg/cmd/go/internal/generate/:

  To convey to humans and machine tools that code is generated,
  generated source should have a line early in the file that
  matches the following regular expression (in Go syntax):

        ^// Code generated .* DO NOT EDIT\.$

src/idl_gen_go.cpp

@@ -782,7 +782,7 @@ class GoGenerator : public BaseGenerator {
   void BeginFile(const std::string name_space_name, const bool needs_imports,
                  std::string *code_ptr) {
     std::string &code = *code_ptr;
-    code = code + "// " + FlatBuffersGeneratedWarning() + "\n\n";
+    code = code + "// Code generated by the FlatBuffers compiler. DO NOT EDIT.\n\n";
     code += "package " + name_space_name + "\n\n";
     if (needs_imports) {
       code += "import (\n";

tests/MyGame/Example/Ability.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example
 

tests/MyGame/Example/Any.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example
 

tests/MyGame/Example/Color.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example
 

tests/MyGame/Example/Monster.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example
 

tests/MyGame/Example/Referrable.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example
 

tests/MyGame/Example/Stat.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example
 

tests/MyGame/Example/Test.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example
 

tests/MyGame/Example/TestSimpleTableWithEnum.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example
 

tests/MyGame/Example/TypeAliases.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example
 

tests/MyGame/Example/Vec3.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example
 

tests/MyGame/Example2/Monster.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package Example2
 

tests/MyGame/InParentNamespace.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package MyGame
 

tests/namespace_test/NamespaceA/NamespaceB/EnumInNestedNS.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package NamespaceB
 

tests/namespace_test/NamespaceA/NamespaceB/StructInNestedNS.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package NamespaceB
 

tests/namespace_test/NamespaceA/NamespaceB/TableInNestedNS.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package NamespaceB
 

tests/namespace_test/NamespaceA/SecondTableInA.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package NamespaceA
 

tests/namespace_test/NamespaceA/TableInFirstNS.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package NamespaceA
 

tests/namespace_test/NamespaceC/TableInC.go

-// automatically generated by the FlatBuffers compiler, do not modify
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
 
 package NamespaceC