Backport PR pandas-dev#39314: DOC: clarify in which version the excel…

… engine default changed (pandas-dev#39322)

Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>

doc/source/user_guide/io.rst

@@ -2834,7 +2834,7 @@ See the :ref:`cookbook<cookbook.excel>` for some advanced strategies.
    The `xlrd <https://xlrd.readthedocs.io/en/latest/>`__ package is now only for reading
    old-style ``.xls`` files.
 
-   Previously, the default argument ``engine=None`` to :func:`~pandas.read_excel`
+   Before pandas 1.2.0, the default argument ``engine=None`` to :func:`~pandas.read_excel`
    would result in using the ``xlrd`` engine in many cases, including new
    Excel 2007+ (``.xlsx``) files.
    If `openpyxl <https://openpyxl.readthedocs.io/en/stable/>`__  is installed,