cairo2 0.5 is not compatible with ocaml 5
```
=== ERROR while compiling cairo2.0.5 =========================================#
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/cairo2.0.5
command ~/.opam/opam-init/hooks/sandbox.sh build ocaml setup.ml -configure --prefix /home/opam/.opam/5.0 --disable-lablgtk2
exit-code 2
env-file ~/.opam/log/cairo2-6-d959c9.env
output-file ~/.opam/log/cairo2-6-d959c9.out
File "./setup.ml", line 316, characters 20-36:
316 | String.compare (String.lowercase s1) (String.lowercase s2)
^^^^^^^^^^^^^^^^
Error: Unbound value String.lowercase
```
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>