Fix MPL warning suppression (#1368)

#1362 didn't work as Matplotlib emits warnings through the `logging`
module rather than the `warnings` module. This follows advice
(matplotlib/matplotlib#23326 (comment))
to correctly suppress the warnings.

scripts/nb-tester/qiskit_docs_notebook_tester/__init__.py

@@ -33,8 +33,9 @@
 
 # We always run the following code in the kernel before running the notebook
 PRE_EXECUTE_CODE = """\
-import warnings
-warnings.filterwarnings("ignore", message="Matplotlib is building the font cache; this may take a moment.")
+import matplotlib
+# See https://github.com/matplotlib/matplotlib/issues/23326#issuecomment-1164772708
+matplotlib.set_loglevel("critical")
 """
 
 # If not submitting jobs, we also run this code before notebook execution to mock the real backend