Give a warning on CLI if GDAL < 3.x is used

[nuest]

When geoextent is executed using GDAL 2.x, there should be a warning log because we cannot guarantee correct coordinate order for GDAL 2.

• [ ] implement test case that only runs for GDAL 2.x; the test should check that there is a severe warning that coordinate order might be switched
• [ ] add a test that only runs when GDAL 3 is used checks there is no warning
• [ ] create GitHub action test environment with latest GDAL 2.x from ubuntugis unstable to run the test
• [ ] add GDAL version to the output of geoextent --version
• [ ] add a debug-level log statement when geoextent is starting that logs the GDAL version

[SbastianGarzon]

This discussion is relevant : https://github.com/OSGeo/gdal/issues/1546