Add backwards compatibility support for setting variable-length point ranges

[kounelisagis]

The daily tests failed last night due to two reasons:

• In libtiledb versions prior to 2.28, the C API tiledb_subarray_add_point_ranges_var wasn't available.
• In libtiledb version 2.28.0, the above C API didn't work as expected. This issue was fixed in Fix Subarray::add_point_ranges_var to use the appropriate variable data Range constructor. TileDB#5560, which was included in version 2.28.1. As a result, the test_add_var_sized_point_ranges test was failing.

To resolve this, there are two options:

1. Do not compile code that uses this C API for versions prior to 2.28.1 - and don't run any tests, or
2. Provide a backwards compatibility solution for earlier versions that doesn't rely on this API.

This PR implements (2). The run of the daily tests on this branch: https://github.com/TileDB-Inc/TileDB-Py/actions/runs/16590958000

Closes #2225