Given a cookie header like "Cookie: foo[]=bar;foo[]=bar2', PHP will
assign $_COOKIE['foo'] = [ 'bar','bar2']. This is not expected by
callers in MediaWiki and can trigger various errors in code that does
not expect to be dealing with arrays in this situation, nor is it
specified by RFC 6265[1] or any of its predecessors. So, stick to RFC
semantics here and treat the cookie as absent, since the name foo[] is
not the same as foo.
[1] https://datatracker.ietf.org/doc/html/rfc6265
Bug: T363980
Change-Id: Idaa175234de2d25a90e6be8fb6279c39089726bd
530b6a408b