release 3.2: Qt6/QGIS 4 support and bug fixes

[fgianoli]

release 3.2: Qt6/QGIS 4 support, native setDataSource, SQL view & provider fixes

• Dual Qt5/Qt6 compatibility via qgis.PyQt + qualified enums (qgisMinimumVersion=3.20)
• Replace XML-based datasource patching with native QgsMapLayer.setDataSource():
  preserves auxiliary storage (manual label positions, data-defined overrides)
  and clears layer tree warning/memory indicators after reload
• Fix StopIteration on long PostGIS SQL views (WITH/window functions):
  memory layer fields now string(0) unlimited instead of string(250)
• Add missing providers: mssql, hana, db2, wfs, arcgisfeatureserver (vector);
  wcs, arcgismapserver, xyz (raster)
• Qt6 API fixes: fontMetrics().horizontalAdvance(), exec(), int coords in move()
• Replace deprecated QgsMessageBar.CRITICAL with Qgis.MessageLevel.Critical
• Use QgsWkbTypes.geometryDisplayString() for safer geometry type resolution
• Null/None guards on locale, mapLayer lookups, expression results
• Drop dead code (QgsMapLayerRegistry, qgisVersionOk), debug prints,
  Python 2 / PyQt4 shims, unused resources_rc import