promis/plugin/check_free_disk_space: np.float -> float

Running tests with recent numpy (1.26.0) yields the following:

    slapos.toolbox$ python -m unittest discover -v   -s slapos/test/promise/plugin/ -t . -k test_display_prediction
    ...
    Enable to display disk space predictions: True
    module 'numpy' has no attribute 'float'.
    `np.float` was a deprecated alias for the builtin `float`. To avoid this error in existing code, use `float` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.float64` here.
    The aliases was originally deprecated in NumPy 1.20; for more details and guidance see the original release note at:
        https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/slapos/slapos.core/slapos/grid/promise/generic.py", line 500, in run
        self.sense()
      File "/home/kirr/src/wendelin/slapos/slapos.toolbox/slapos/promise/plugin/check_free_disk_space.py", line 327, in sense
        disk_space_prediction_tuple = self.diskSpacePrediction(
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/home/kirr/src/wendelin/slapos/slapos.toolbox/slapos/promise/plugin/check_free_disk_space.py", line 182, in diskSpacePrediction
        df = df.astype({'free': np.float})
                                ^^^^^^^^
      File "/home/kirr/src/wendelin/venv/py3.venv/lib/python3.11/site-packages/numpy/__init__.py", line 324, in __getattr__
        raise AttributeError(__former_attrs__[attr])
    AttributeError: module 'numpy' has no attribute 'float'.
    `np.float` was a deprecated alias for the builtin `float`. To avoid this error in existing code, use `float` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.float64` here.
    The aliases was originally deprecated in NumPy 1.20; for more details and guidance see the original release note at:
        https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
    ERROR

-> Replace np.float with float as the error says.

/cc @Just1, @lu.xu, @jhuge, @tomo
/reviewed-on !123