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.