[Intl] Remove warning about replacement layer

[smnandre]

The "replacement layer" mentionned in the first block on the page has been deprecated in 5.3 (and removed in 6.0).

(that's why i target 6.3+ and not 5.4 -- where it still makes sense to mention it)

.. caution::

    The replacement layer is limited to the ``en`` locale. If you want to use
    other locales, you should `install the intl extension`_. There is no conflict
    between the two because, even if you use the extension, this package can still
    be useful to access the ICU data.

[javiereguiluz]

Thanks Simon.