Auto merge of #124773 - Marcondiro:master, r=joboet

fix #124714 str.to_lowercase sigma handling

Hello,
This PR fixes issue #124714 about 'Σ' handling in `str.to_lowercase()`.
The fix consists in considering the full original string during 'Σ' handling instead of considering just the substring left after the optimized ascii handling.
A new test is added to avoid regression.
Thanks!

Author: bors