MAINT: drop support for GDAL 3.4 and 3.5

[theroggy]

As discussed in geopandas community meeting of 2025-09-25.

The first version of GDAL 3.6 dates already from 2022-12-11, so already almost 3 years old at the time of writing, so dropping older versions should be fine.

For reference: Scientific Python - SPEC 0 advises:

• 3 year support for python versions after initial release
• 2 year support for core package dependencies after initial release