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>

After the type registration changes, importing a style explicitly
will cause that style's QML types to be used, so applications
should put style-specific code into file-selected directories if
they need to support multiple styles.

[ChangeLog][Important Behavior Changes] Due to the recent type
registration changes, importing a style explicitly (e.g.
"import QtQuick.Controls.Material") now registers that style's
QML types in addition to making its API (attached, singleton, etc.)
available. For this reason, it is now advised to have style-specific
code in a separate QML file and use file selectors if your application
supports more than one style. If your style only supports one style,
importing that style explicitly will work as expected.
For example, if you use Material.foreground in your QML code and your
application supports more than one style, you should refactor the code
that uses the binding into its own file; e.g.
+Material/MyComponent.qml.

Fixes: QTBUG-86263
Change-Id: I38e40ff4f20f61218550ad73945dafb912193466
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>