Qt Quick Controls 2 was named that way because it was a follow-up to
Qt Quick Controls 1.x. Now that Qt Quick Controls 1 is no longer
supported, we don't need to have "2" in the name. Work on this was
already started for the documentation in
1abdfe5d5a052f2298b7bf657513dfa7e0c66a56.

By doing this renaming a few weeks before feature freeze, it won't
affect the release but still results in as little time possible spent
manually fixing conflicts in cherry-picks from non-LTS releases as a
result of the renaming.

This patch does the following:

- Renames directories.
- Adapts CMakeLists.txt and other files to account for the new paths.

A follow-up patch will handle documentation.

It does not touch library names or other user-facing stuff, as that
will have to be done in Qt 7.

Task-number: QTBUG-95413
Change-Id: I170d8db19033ee71e495ff0c5c1a517a41ed7634
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>