TST: bump mpldev image test references - #19562

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These changes should all be due to matplotlib/matplotlib#30161, which overhauled how fonts are rendered. I guess there might be image differences beyond just text if the text changes happen to affect bounds calculations for a given plot.

Excellent. Thanks a lot !
I took another look and

• all diff seem purely textual
• they're not always as subtle as a single pix diff
• the biggest one is the following