patch 9.1.0709: GUIEnter event not found in Turkish locale

patch 9.1.0709: GUIEnter event not found in Turkish locale

Problem:   GUIEnter not found in Turkish locale
           (James McCoy, after v9.1.0256, the issue was there before,
            but v9.1.0256 made it more apparent)
Solution:  explicitly compare autocommand events by ASCII value and
           ignoring locale, because according to the documentation,
           events are case insensitive (:h autocommand-events)

fixes: #15574
closes: #15603

Signed-off-by: Christian Brabandt <cb@256bit.org>