libstdc++: Only define std::span::at for C++26 [PR115335]

In r14-5689-g1fa85dcf656e2f I added std::span::at and made the correct
changes to the __cpp_lib_span macro (with tests for the correct value in
C++20/23/26). But I didn't make the declaration of std::span::at
actually depend on the macro, so it was defined for C++20 and C++23, not
only for C++26. This fixes that oversight.

libstdc++-v3/ChangeLog:

	PR libstdc++/115335
	* include/std/span (span::at): Guard with feature test macro.

(cherry picked from commit 2197814011eec75022aa8550f10621409b69d4a1)

libstdc++-v3/include/std/span

@@ -287,6 +287,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *(this->_M_ptr + __idx);
       }
 
+#if __cpp_lib_span >= 202311L // >= C++26
       [[nodiscard]]
       constexpr reference
       at(size_type __idx) const
@@ -296,6 +297,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				       "of size %zu"), __idx, this->size());
 	return *(this->_M_ptr + __idx);
       }
+#endif
 
       [[nodiscard]]
       constexpr pointer