chore: revert Concrete Python to 2.6.0-rc1

[RomanBredehoft]

their release was not complete (in particular, it was lacking python 3.9 support on ubuntu)

[github-actions]

Coverage failed ❌

Coverage details

---------- coverage: platform linux, python 3.8.18-final-0 -----------
Name                                                  Stmts   Miss  Cover   Missing
-----------------------------------------------------------------------------------
src/concrete/ml/onnx/onnx_model_manipulations.py        101      2    98%   45, 48
src/concrete/ml/pandas/__init__.py                        8      2    75%   19, 81
src/concrete/ml/pandas/_development.py                   42     20    52%   30, 64-71, 83, 99-106, 131, 142, 155-171
src/concrete/ml/pandas/_operators.py                     75     65    13%   35-38, 57-90, 135-238, 310-374
src/concrete/ml/pandas/_processing.py                    86     72    16%   27-52, 66, 83-85, 111-191, 211-228, 246-290, 310-321
src/concrete/ml/pandas/_utils.py                         36     20    44%   28-36, 52, 69-70, 83-84, 96, 109, 121, 133, 145, 157, 174-177, 190
src/concrete/ml/pandas/client_engine.py                  27     13    52%   20-23, 34-37, 48-60, 79-85
src/concrete/ml/pandas/dataframe.py                      98     59    40%   39-50, 59, 68, 77, 86, 95, 104, 113, 123-131, 142-143, 154-158, 168, 228-256, 266-283, 297-306, 321-332, 344-357
src/concrete/ml/pytest/torch_models.py                  614      8    99%   1330-1333, 1344-1347
src/concrete/ml/pytest/utils.py                         171     21    88%   508-517, 558, 560, 717-755
src/concrete/ml/quantization/quantized_module.py        222      3    99%   229, 250, 265
src/concrete/ml/search_parameters/p_error_search.py     122     96    21%   106-146, 209-228, 233-239, 245-246, 255-266, 270-271, 291-300, 332-363, 374-380, 425-514
-----------------------------------------------------------------------------------
TOTAL                                                  7548    381    95%

47 files skipped due to complete coverage.