Dropping Python 3.9 support in conda-forge

With Python 3.9 reaching end-of-life in Oct 2025 and Python 3.14 being released the same month, we have decided to drop 3.9 from our default build matrix. This will be reflected in your feedstock configuration on the next rerender. The decision to drop support 1.5 months before its EOL is to avoid the strain on conda-forge CI while we add support for 3.14.