Added breaking change note for @export_file behavior in 4.4

[Ayush41]

This PR updates the "Upgrading to Godot 4.4" documentation to include a breaking change notice for the @export_file annotation.

As of Godot 4.4, setting a value via the Inspector stores it as a uid:// reference instead of the familiar res:// path. This causes mixed path formats and may break assumptions in GDScript or serialized .tscn/.tres files.

Based on issue #11065

Details

• In 4.4, @export_file behavior changed, and there's no built-in way to retain res:// paths from the Inspector.
• Users must manually edit .tscn or .tres files to preserve the old path style.
• In 4.5, a new annotation @export_file_path was introduced to restore the original behavior explicitly.

This documentation update helps users understand:

• Why their exported paths changed.
• How to work around it in 4.4.
• What to use in 4.5+ to restore the previous behavior.

References

• Fixes: "addons" folder should be renamed to "plugins" godot#11065
• Related PR (4.5): Add @export_file_path to export raw paths (no UID) godot#105414
• UID storage changes: Array of files is saved as uids in ProjectSettings since 4.4 godot#104818

Let me know if this needs formatting changes or should be placed in a different section.