make gdal dep optional

[iboates]

GDAL can be very cumbersome to install, since it is a Python wrapper to a system binary. That system binary is also notoriously difficult to deal with, as newer versions of Linux (at least Ubuntu) stop adding newer versions to old apt repositories. As a result, it can cause serious headaches with dependency management. It gets even worse when using stricter package management systems, like poetry.

As far as I can tell, this library only actually needs GDAL for one function (create_coveragestyle). This PR removes GDAL from requirements.txt and raises a custom ImportError when the GDAL-specific code is invoked.

BONUS FIX: Apparently the Github action pipelines were failing, too, because the ubuntugis ppa is incompatible with the latest ubuntu version. I pegged the test pipeline to use ubuntu-22.04, it works for now, although it certainly won't stay that way forever.

[iamtekson]

This looks good. Maybe its a good time to update the docs as well. I think we need to inform user about this changes.

Can you please update the documentation how to use.rst file's section "Creating and applying dynamic styles based on the raster coverages"?

Also please update the version number. It should be 2.9

[iboates]

This looks good. Maybe its a good time to update the docs as well. I think we need to inform user about this changes.

Can you please update the documentation how to use.rst file's section "Creating and applying dynamic styles based on the raster coverages"?

Also please update the version number. It should be 2.9

All done!

On the topic of docs, the API docs have been non-functional for quite a while now, as per https://geoserver-rest.readthedocs.io/en/latest/geo.html