Move qtformat_impl.h into corelib/text

... because it is not really related to IO.
We were choosing between serialization and text, and ended up with
text, because that's where the standard itself puts format [0].

[0]: https://eel.is/c++draft/format.functions

Amends ffac33964d51971a1c1f61308b6f0a60a4133296.

Found in Qt 6.9 API review.

Pick-to: 6.9
Change-Id: I827322175da81bc027a9e136098b7e84677f43f7
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/CMakeLists.txt

@@ -144,7 +144,6 @@ qt_internal_add_module(Core
         io/qstorageinfo.cpp io/qstorageinfo.h io/qstorageinfo_p.h
         io/qtemporarydir.cpp io/qtemporarydir.h
         io/qtemporaryfile.cpp io/qtemporaryfile.h io/qtemporaryfile_p.h
-        io/qtformat_impl.h
         io/qurl.cpp io/qurl.h io/qurl_p.h
         io/qurlidna.cpp
         io/qurlquery.cpp io/qurlquery.h
@@ -243,6 +242,7 @@ qt_internal_add_module(Core
         text/qstringtokenizer.cpp text/qstringtokenizer.h
         text/qstringview.cpp text/qstringview.h
         text/qtextboundaryfinder.cpp text/qtextboundaryfinder.h
+        text/qtformat_impl.h
         text/qunicodetables_p.h
         text/qunicodetools.cpp text/qunicodetools_p.h
         text/qutf8stringview.h