qt6_add_translations() deletes CMake-generated *.ts files when project is cleaned

If a *.ts file in qt6_add_translations() does not exist, the build system kindly offers to auto-generate it:

CMake Warning at C:/Qt/6.4.0/msvc2019_64/lib/cmake/Qt6LinguistTools/Qt6LinguistToolsMacros.cmake:241 (message):
  Translation file
  'C:/Qt/Examples/Qt-6.4.0/linguist/hellotr/hellotr_la.ts' does not
  exist.  Consider building the target 'hellotr_lupdate' to create an initial
  version of that file.

Unfortunately, auto-generated files also get auto-deleted. 

Steps to reproduce

1. In Qt Creator, load the CMakeLists.txt file for the Hello tr() Example (https://doc.qt.io/qt-6/qtlinguist-hellotr-example.html ).
2. Build the `update translations` target to auto-generate hellotr_la.ts.
3. Use Qt Linguist to translate hellotr_la.ts
4. Build the `hellotr` target and run the executable to confirm that the translation is applied correctly
5. In Qt Creator, click "Build" > "Clean"

Actual Outcome

At the end of Step #5, hellotr_la.ts is deleted. The translator's work is lost.

Expected Outcome

At the end of Step #5, hellotr_la.ts and its contents are preserved.

Workaround

Ensure that all of your project's required *.ts files already exist prior to building your project for the first time. CMake ... --target clean won't delete files that CMake did not generate.