upgrade gpxpy to be timezone aware

[ptpt]

NOTE: it's a breaking change upgrading gpxpy from 1.5 -> 1.6

This pull request focuses on dependency updates, minor code cleanup, and test data consistency improvements. The main highlights are the modernization of dependency version specifications, the removal of an unused class, and updates to test GPX timestamps for stricter ISO 8601 compliance.

Dependency updates and modernization:

• Updated dependency version specifications in pyproject.toml to use ~= (compatible release) and allow newer versions for construct, exifread, gpxpy, jsonschema, and piexif. This improves compatibility with newer package releases and simplifies future maintenance.

Code cleanup:

• Removed the unused SyncMode class from geotag_images_from_gpx.py, reducing unnecessary code and potential confusion.

Test data consistency:

• Updated GPX trackpoint timestamps in test_process.py to include the trailing Z, ensuring timestamps are in strict ISO 8601 format and improving test reliability.