GB-066 12.4.5p1 [over.sub] Accept "P2589R0 static operator[]" for consistency with the static operator()

[wg21bot]

We've accepted static operator() without the suggested fixup to the operator[] for symmetry.

The paper is ready to be accepted and I see no reason to wait 3 years and have a mismatch in the language because the paper could only be submitted after both operator[] has been accepted and operator() was made static.

See P2589R0.

[jfbastien]

Potential duplicate of #460

[erichkeane]

No objection to unanimous consent that this is a duplicate as stated above.