Fix [C#] Object API - Invalid Property Name used in UnPackTo for unio… (#7751)

* Fix [C#] Object API - Invalid Property Name used in UnPackTo for union fieldhttps://github.com/google/flatbuffers/issues/7750, also fixes invalid Code generated in WriteJson for Unions named Value.

* Test added: new schema union_value_collision.fbs with a Union named Value and a union field named value. The generated C# code now compiles when NetTest.bat. The Code generated with an older flatc.exe didn't compile because of a mismatch of the property name (Value vs. Value_).

* branch was not up-to-date with master

* BASE_OPTS + CPP_OPTS removed and union_value_collision_generated.h deleted

Co-authored-by: Derek Bailey <derekbailey@google.com>

tests/union_value_collision.fbs

@@ -0,0 +1,17 @@
+namespace union_value_collsion;
+
+table IntValue {
+	value:int;
+}
+
+union Value { IntValue }
+
+union Other { IntValue }
+
+// This table tests collsions of Unions and fields named value.
+table Collision {
+  some_value : Value;
+  value : Other;
+}
+
+root_type Collision;