Deprecate `\oldcode` and `\newcode`

QDoc provides a pair of commands, oldcode and newcode, that are used "to show how to port a snippet of code to a new version of an API".

A usage of the commands-pair resolves to the equivalent of two code-blocks with some text before the first code-block and in-between the two-codeblocks.

For example:

\oldcode
    if (printer->setup(parent))
        ...
\newcode
    QPrintDialog dialog(printer, parent);
        if (dialog.exec())
            ...
\endcode

Would be equivalent to:

For example, if you have code like

\code
    if (printer->setup(parent))
        ...
\endcode

you can rewrite it as

\code
    QPrintDialog dialog(printer, parent);
        if (dialog.exec())
            ...
\endcode

This feature isn't particularly used.
Indeed, only 15 cases were identified in the current documentation across Qt.

As the feature has a low usage statistic while being of an arguable utility considering the triviality of producing a similar effect manually, it was marked for removal.

Hence, deprecate the feature in QDoc and produce a warning for its usage notifying the user that it will be removed in the near future.

Furthermore, the known usage should be converted to the equivalent non \oldcode-\newcode version.