rtree/test_rtree.hpp uses std::allocator rebind deprecated in C++17 and removed in C++20

[yuxianch]

rebind in std::allocator is deprecated in C++17 and removed in C++20. See https://en.cppreference.com/w/cpp/memory/allocator.

This change makes that the tests that include test_rtree.hpp are failed in C++20 since rebind is used in serveral lines of test_rtree.hpp. https://github.com/boostorg/geometry/blob/18f6ad3658b987ff96e8e2b2d2a752584e5e74cb/index/test/rtree/test_rtree.hpp#L1697

The use of rebind in test_rtree.hpp needs to be modified to conform to C++20.

[mloskot]

@yuxianch Pull requests are always welcome