Deprecates $wgRateLimitLog in favor of debug log
$wgRateLimitLog is meant to log rate limiting occuring in MediaWiki. The
feature was introduced by
e46d19e72d3 / r9202 in 2005. We might not
have wfDebugLog() at that time and we used error_log() with the global
pointing to a file. A later patch converted to file_put_contents().
On Wikimedia production the variable is set to a udp:// URL, I am not
sure file_put_contents() can write to it. Additionally the warning are
suppressed which I tend to hate.
Instead use the log bucket 'ratelimit'.
For backcompabitilibity purposes, if $wgRateLmitLog is still set:
attempt to set $wgDebugLogGroup['ratelimit'] with its value.
Change-Id: I86131c4a8053800afd9f41fa1906c998a88736c6