CI: Temporarily pin to numpy<2 in the "GMT Dev Tests" workflow

[seisman]

The "GMT Dev Tests" workflow is failing likely because cftime is incompatible with numpy 2.x (Unidata/cftime#325):

A module that was compiled using NumPy 1.x cannot be run in
NumPy 2.1.0.dev0 as it may crash. To support both 1.x and 2.x
versions of NumPy, modules must be compiled with NumPy 2.0.
Some module may need to rebuild instead e.g. with 'pybind11>=2.12'.

If you are a user of the module, the easiest solution will be to
downgrade to 'numpy<2' or try to upgrade the affected module.
We expect that some modules will need time to support NumPy 2.

Traceback (most recent call last):  File "<string>", line 1, in <module>
  File "/home/runner/work/pygmt/pygmt/pygmt/__init__.py", line 30, in <module>
    from pygmt import datasets
  File "/home/runner/work/pygmt/pygmt/pygmt/datasets/__init__.py", line 7, in <module>
    from pygmt.datasets.earth_age import load_earth_age
  File "/home/runner/work/pygmt/pygmt/pygmt/datasets/earth_age.py", line 10, in <module>
    from pygmt.datasets.load_remote_dataset import _load_remote_dataset
  File "/home/runner/work/pygmt/pygmt/pygmt/datasets/load_remote_dataset.py", line 9, in <module>
    from pygmt.clib import Session
  File "/home/runner/work/pygmt/pygmt/pygmt/clib/__init__.py", line 8, in <module>
    from pygmt.clib.session import Session
  File "/home/runner/work/pygmt/pygmt/pygmt/clib/session.py", line 17, in <module>
    import xarray as xr
  File "/home/runner/micromamba/envs/pygmt/lib/python3.12/site-packages/xarray/__init__.py", line 3, in <module>
    from xarray import testing, tutorial
  File "/home/runner/micromamba/envs/pygmt/lib/python3.12/site-packages/xarray/testing/__init__.py", line 2, in <module>
    from xarray.testing.assertions import (  # noqa: F401
  File "/home/runner/micromamba/envs/pygmt/lib/python3.12/site-packages/xarray/testing/assertions.py", line 12, in <module>
    from xarray.core.coordinates import Coordinates
  File "/home/runner/micromamba/envs/pygmt/lib/python3.12/site-packages/xarray/core/coordinates.py", line [16](https://github.com/GenericMappingTools/pygmt/actions/runs/8871676257/job/24354984638#step:12:17), in <module>
    from xarray.core.alignment import Aligner
  File "/home/runner/micromamba/envs/pygmt/lib/python3.12/site-packages/xarray/core/alignment.py", line 24, in <module>
    from xarray.core.variable import Variable, as_compatible_data, calculate_dimensions
  File "/home/runner/micromamba/envs/pygmt/lib/python3.12/site-packages/xarray/core/variable.py", line [19](https://github.com/GenericMappingTools/pygmt/actions/runs/8871676257/job/24354984638#step:12:20), in <module>
    from xarray.core import common, dtypes, duck_array_ops, indexing, nputils, ops, utils
  File "/home/runner/micromamba/envs/pygmt/lib/python3.12/site-packages/xarray/core/common.py", line 26, in <module>
    import cftime
  File "/home/runner/micromamba/envs/pygmt/lib/python3.12/site-packages/cftime/__init__.py", line 1, in <module>
    from ._cftime import (datetime, real_datetime,

[seisman]

It's unclear when NumPy 2.x and the compatible cftime will be released. This PR pins to NumPy<2 in the "GMT Dev Tests" workflow so the workflow won't fail due to incompatible packages.

Later, I'll open a separate PR to revert the changes in this PR so that we can track if there are any changes in numpy or cftime.