pygmt.show_versions: Report GDAL version by calling the GDAL C function

[seisman]

Description of proposed changes

Query the GDAL version by calling the GDAL C function GDALVersionInfo (https://gdal.org/en/stable/api/raster_c_api.html#_CPPv415GDALVersionInfoPKc).

Fixes #4102.

[Copilot]

Pull Request Overview

This PR improves GDAL version reporting in PyGMT by querying the GDAL version directly from the GDAL C library using ctypes instead of relying on Python package metadata.

• Adds a new _get_gdal_version() function that uses ctypes to call the GDAL C API's GDALVersionInfo function
• Updates the dependency information collection to use the new GDAL version function instead of treating GDAL as a regular Python package
• Adds test assertions to verify that GDAL version is properly reported and not returning None

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
pygmt/_show_versions.py	Implements new GDAL version detection using ctypes and updates dependency collection logic
pygmt/tests/test_show_versions.py	Adds test coverage to verify GDAL version is correctly reported

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