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