LWG-4135 The helper lambda of std::erase for list should specify return type as bool

[StephanTLavavej]

LWG-4135 The helper lambda of std::erase for list should specify return type as bool

This was almost fully implemented by #4900, but the issue resolution also added const which we don't have:

STL/stl/inc/forward_list

Lines 1618 to 1619 in 5e0ddad

	forward_list<_Ty, _Alloc>::size_type erase(forward_list<_Ty, _Alloc>& _Cont, const _Uty& _Val) {
	return _Cont.remove_if([&](_Ty& _Elem) -> bool { return _Elem == _Val; });

STL/stl/inc/list

Lines 1923 to 1924 in 5e0ddad

	list<_Ty, _Alloc>::size_type erase(list<_Ty, _Alloc>& _Cont, const _Uty& _Val) {
	return _Cont.remove_if([&](_Ty& _Elem) -> bool { return _Elem == _Val; });