Old version of cow does not work with OCaml 5

It fails with:
```
  #=== ERROR while compiling cow.2.2.0 ==========================================#
  # context              2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.5.0.0 | file:///home/opam/opam-repository
  # path                 ~/.opam/5.0/.opam-switch/build/cow.2.2.0
  # command              ~/.opam/opam-init/hooks/sandbox.sh build ocaml pkg/pkg.ml build --pinned false
  # exit-code            1
  # env-file             ~/.opam/log/cow-7-2576fb.env
  # output-file          ~/.opam/log/cow-7-2576fb.out
  ### output ###
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_atom.ml > src/cow_atom.ml.depends
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_atom.mli > src/cow_atom.mli.depends
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_xml.mli > src/cow_xml.mli.depends
  # ocamlfind ocamlc -c -g -annot -bin-annot -safe-string -principal -strict-sequence -package uri -package xmlm -package ezjsonm -package omd -package bytes -I src -I test -o src/cow_xml.cmi src/cow_xml.mli
  # ocamlfind ocamlc -c -g -annot -bin-annot -safe-string -principal -strict-sequence -package uri -package xmlm -package ezjsonm -package omd -package bytes -I src -I test -o src/cow_atom.cmi src/cow_atom.mli
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_xml.ml > src/cow_xml.ml.depends
  # ocamlfind ocamlopt -c -g -annot -bin-annot -safe-string -principal -strict-sequence -package uri -package xmlm -package ezjsonm -package omd -package bytes -I src -I test -o src/cow_xml.cmx src/cow_xml.ml
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_html.ml > src/cow_html.ml.depends
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_html.mli > src/cow_html.mli.depends
  # ocamlfind ocamlc -c -g -annot -bin-annot -safe-string -principal -strict-sequence -package uri -package xmlm -package ezjsonm -package omd -package bytes -I src -I test -o src/cow_html.cmi src/cow_html.mli
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_xhtml.ml > src/cow_xhtml.ml.depends
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_xhtml.mli > src/cow_xhtml.mli.depends
  # ocamlfind ocamlc -c -g -annot -bin-annot -safe-string -principal -strict-sequence -package uri -package xmlm -package ezjsonm -package omd -package bytes -I src -I test -o src/cow_xhtml.cmi src/cow_xhtml.mli
  # ocamlfind ocamlopt -c -g -annot -bin-annot -safe-string -principal -strict-sequence -package uri -package xmlm -package ezjsonm -package omd -package bytes -I src -I test -o src/cow_xhtml.cmx src/cow_xhtml.ml
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_json.ml > src/cow_json.ml.depends
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_json.mli > src/cow_json.mli.depends
  # ocamlfind ocamlc -c -g -annot -bin-annot -safe-string -principal -strict-sequence -package uri -package xmlm -package ezjsonm -package omd -package bytes -I src -I test -o src/cow_json.cmi src/cow_json.mli
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_markdown.ml > src/cow_markdown.ml.depends
  # ocamlfind ocamldep -package uri -package xmlm -package ezjsonm -package omd -package bytes -modules src/cow_markdown.mli > src/cow_markdown.mli.depends
  # ocamlfind ocamlc -c -g -annot -bin-annot -safe-string -principal -strict-sequence -package uri -package xmlm -package ezjsonm -package omd -package bytes -I src -I test -o src/cow_markdown.cmi src/cow_markdown.mli
  # ocamlfind ocamlopt -c -g -annot -bin-annot -safe-string -principal -strict-sequence -package uri -package xmlm -package ezjsonm -package omd -package bytes -I src -I test -o src/cow_html.cmx src/cow_html.ml
  # + ocamlfind ocamlopt -c -g -annot -bin-annot -safe-string -principal -strict-sequence -package uri -package xmlm -package ezjsonm -package omd -package bytes -I src -I test -o src/cow_html.cmx src/cow_html.ml
  # File "src/cow_html.ml", line 411, characters 12-28:
  # 411 |     let s = String.lowercase s in
  #                   ^^^^^^^^^^^^^^^^
  # Error: Unbound value String.lowercase
```

Author: samoht

packages/cow/cow.2.2.0/opam

@@ -17,7 +17,7 @@ tags: [
 dev-repo: "git+https://github.com/mirage/ocaml-cow.git"
 build: ["ocaml" "pkg/pkg.ml" "build" "--pinned" "%{pinned}%"]
 depends: [
-  "ocaml" {>= "4.02.3"}
+  "ocaml" {>= "4.02.3" & < "5.0"}
   "ocamlfind" {build}
   "ocamlbuild" {build}
   "topkg" {build}