Warn about PyPy being unmaintained (#17643)

It seems that PyPy is not being actively developed anymore and is phased
out even by numpy (https://github.com/numpy/numpy/issues/30416). There's
no official statement from the project, but the numpy issue is from a
PyPy developer. I added a warning to avoid users assuming PyPy properly
supported and developed Python distribution, and in anticipation of PyPy
being eventually, slowly deprecated.

---------

Co-authored-by: Tomasz Kramkowski <tom@astral.sh>

docs/concepts/python-versions.md

@@ -470,6 +470,11 @@ documentation for details.
 
 ### PyPy distributions
 
+!!! note
+
+    PyPy is [not actively developed anymore](https://github.com/numpy/numpy/issues/30416) and
+    only supports Python versions up to 3.11
+
 PyPy distributions are provided by the [PyPy project](https://pypy.org).
 
 ### Pyodide distributions