Update knowledge_base.md

The matplotlib feedstock changed the qt wrapper from pyqt to pyside6. So the hint towards the pyqt requirement is outdated.
See this commit:
conda-forge/matplotlib-feedstock@330c36e

Author: HLinde

docs/maintainer/knowledge_base.md

@@ -1397,10 +1397,10 @@ package will have the necessary runtime requirements to ensure the most adequate
 ### Matplotlib
 
 `matplotlib` on conda-forge comes in two parts. The core library is in `matplotlib-base`. The
-actual `matplotlib` package is this core library plus `pyqt`. Most, if not all, packages that have
+actual `matplotlib` package is this core library plus `pyside6`. Most, if not all, packages that have
 dependence at runtime on `matplotlib` should list this dependence as `matplotlib-base` unless they
-explicitly need `pyqt`. The idea is that a user installing `matplotlib` explicitly would get a full
-featured installation with `pyqt`. However, `pyqt` is a rather large package, so not requiring it
+explicitly need `pyside6`. The idea is that a user installing `matplotlib` explicitly would get a full
+featured installation with `pyside6`. However, `pyside6` is a rather large package, so not requiring it
 indirectly is better for performance. Note that you may need to include a `yum_requirements.txt` file
 in your recipe with