qt6.m4 sets QT_DISABLE_DEPRECATED_BEFORE below Qt6
Qt6 still seems to support some things deprecated in 6.0.0 -> we can't simply bump the QT_DISABLE_DEPRECATED_BEFORE without some code changes.
We probably don't want to start making those changes to S3_1.
For Qt6 compilation QT_DISABLE_DEPRECATED_BEFORE should obviously be at least 6.0. qt6.m4 currently sets it to 0x050f00