Replace QPair with std::pair in the docs

QPair _is_ std::pair. It's confusing that debuggers consistently show
the real name, std::pair, while the API and docs continue to maintain
the illusion that there is such a thing as QPair.

Use std::pair everywhere.

Pick-to: 6.5
Task-number: QTBUG-115841
Change-Id: I009e2fc415a79a74b583a13cf11e4ff9483a7f6b
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
(cherry picked from commit ec67d0c942a7756afbb6e21c76e835efd3557427)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 8cba66e91dcfa9de7c286cbca214d9d5a0ab8669)

src/corelib/doc/snippets/code/src_corelib_serialization_qcborstream.cpp

@@ -183,7 +183,7 @@ using namespace Qt::StringLiterals;
 //! [21]
 
 //! [22]
-   void appendMap(QCborStreamWriter &writer, const QList<QPair<int, QString>> &values)
+   void appendMap(QCborStreamWriter &writer, const QList<std::pair<int, QString>> &values)
    {
        writer.startMap();
        for (const auto pair : values) {

src/corelib/doc/src/datastreamformat.qdoc

@@ -46,7 +46,7 @@
     \li QMap<Key, T>
     \li QMargins
     \li QMatrix4x4
-    \li QPair<T1, T2>
+    \li std::pair<T1, T2>
     \li QPalette
     \li QPen
     \li QPicture

src/corelib/doc/src/foreach-keyword.qdoc

@@ -31,7 +31,7 @@
 
     \snippet code/doc_src_containers.cpp 16
 
-    Unless the data type contains a comma (e.g., \c{QPair<int,
+    Unless the data type contains a comma (e.g., \c{std::pair<int,
     int>}), the variable used for iteration can be defined within the
     \c foreach statement: