Change default of EnforceGenericsUB to 2

Summary:
Fully enforce upper bounds for generic types by default, in preparation for
removal of the option.

Reviewed By: ricklavoie

Differential Revision: D52947697

fbshipit-source-id: f9623feef7196afd1eae20375c1a0a50c243cca9

Author: jano

hphp/runtime/base/runtime-option.h

@@ -865,7 +865,7 @@ struct RuntimeOption {
   /* Enables enforcing upper-bounds for generic types
      1 => warning, 2 => error
   */                                                                    \
-  F(int32_t, EnforceGenericsUB,        1)                               \
+  F(int32_t, EnforceGenericsUB,        2)                               \
   /* WarnOnTooManyArguments:
    * 0 -> no warning, 1 -> warning, 2 -> exception
    */                                                                   \