Document OpenCascade 8 compatibility

[newmattock]

Summary:
Adds a FreeCAD 1.2 release-note entry for FreeCAD/FreeCAD#25502.

Related bounty or issue:
Refs #30.

What changed:

• Documented that FreeCAD was adapted for OpenCascade 8 while keeping OpenCascade 7 compatibility across geometry, import/export, CAM, Sketcher, FEM mesh, and related code paths.
• Kept the update limited to the root Release_notes_1.2 page.

Validation:

• git diff --check -- wiki/Release_notes_1.2.wikitext

Notes:

• Translation files were intentionally not modified.
• This is a focused release-note addition and does not claim the broader release-notes issue is complete.