Support free-threaded Python builds

### Description of the desired feature

Python 3.13 came out with an experimental free-threaded mode with the global-interpreter lock disabled ([PEP 703](https://peps.python.org/pep-0703/)). This issue is to track the addition for PyGMT to build free-threaded compatible wheels.

**Main guide**: https://py-free-threading.github.io/porting/

Steps:
- [ ] Test building PyGMT on Python ~3.13t~ 3.14t in CI
- https://py-free-threading.github.io/ci/
- [ ] Document thread safety guarantees via trove classifiers
- https://py-free-threading.github.io/porting/#define-and-document-thread-safety-guarantees
- [ ] Validate thread safety using [`pytest-freethreaded`](https://github.com/tonybaloney/pytest-freethreaded) or [`pytest-run-parallel`](https://github.com/Quansight-Labs/pytest-run-parallel)
- https://py-free-threading.github.io/testing/

Notes:
- Some [GMT modules already support multi-threading](https://docs.generic-mapping-tools.org/6.5/reference/options.html#option-x-core) via the `-x` flag, which happens via OpenMP, need to be careful that we don't do threading of many PyGMT functions + call multi-threaded GMT functions
- Related thread on multi-processing support - #217
- Dependencies that have added free-threading support
- NumPy (since 2.1) - https://numpy.org/doc/2.3/reference/thread_safety.html
- Pandas (since 2.2.3) - https://pandas.pydata.org/pandas-docs/version/2.2/whatsnew/v2.2.3.html#pandas-2-2-3-is-now-compatible-with-python-3-13

### Are you willing to help implement and maintain this feature?

Yes