Mark Slacko as potentially compatible with Lwt 4.0

According to the nice comment by @aantron in
#11708 (comment)
slacko fails because it expects lwt.log to exist without depending on
the lwt_log message.

This came to a surprise to me since Slacko does not do any logging at
all, so constraining it does not seem to be useful.

Looking in the CI log
https://ci.ocamllabs.io/log/saved/docker-run-e97b0a51ae9b04229ade4297103e6c79/e87df7e1b52e41541b179ab4024a538bfd0b1484
I can see that the error came from a module in "lwt/cohttp_lwt_unix.ml"
which happens when building cohttp.0.21.1.

Therefore I would like to correct the dependency here. If any version of
cohttp < 0.99 would be compatible with Lwt 4 (e.g. if someone does a
maintenance release), so would slacko.

packages/slacko/slacko.0.13.0/opam

@@ -18,7 +18,7 @@ depends: [
   "ocamlfind"
   "cmdliner"
   "yojson"
-  "lwt" {>= "2.4.7" & < "4.0.0"}
+  "lwt" {>= "2.4.7"}
   "tls" | "ssl"
   "cohttp" {>= "0.21.1" & <"0.99"}
   "ppx_deriving_yojson" {>= "3.0"}