Add support for OCaml 5.3

[kit-ty-kate]

See ocaml/opam-repository#27199

As a side note, it required gluon main otherwise gluon would fail to build with:

#=== ERROR while compiling gluon.0.0.9 ========================================#
# context     2.3.0 | linux/arm64 | ocaml-base-compiler.5.3.0~rc1 | https://opam.ocaml.org#631e587f169807aaf233a49289573c1bb7c174b7
# path        ~/.opam/5.3/.opam-switch/build/gluon.0.0.9
# command     ~/.opam/opam-init/hooks/sandbox.sh build dune build -p gluon -j 9 @install
# exit-code   1
# env-file    ~/.opam/log/gluon-125330-f625b8.env
# output-file ~/.opam/log/gluon-125330-f625b8.out
### output ###
# File "gluon/sys/unix/dune", line 11, characters 41-57:
# 11 |   (names gluon_unix_io gluon_unix_kqueue gluon_unix_epoll)
#                                               ^^^^^^^^^^^^^^^^
# (cd _build/default/gluon/sys/unix && /usr/bin/gcc -O2 -fno-strict-aliasing -fwrapv -fPIC -pthread -D_FILE_OFFSET_BITS=64 -fdiagnostics-color=always -O2 -g -I /home/kit_ty_kate/.opam/5.3/lib/ocaml -I /home/kit_ty_kate/.opam/5.3/lib/libc -I /home/kit_ty_kate/.opam/5.3/lib/ocaml/unix -I ../../common -I ../../events -o gluon_unix_epoll.o -c gluon_unix_epoll.c)
# gluon_unix_epoll.c: In function ‘gluon_unix_epoll_ctl’:
# gluon_unix_epoll.c:73:20: error: assignment to ‘uint64_t’ {aka ‘long unsigned int’} from ‘value *’ {aka ‘long int *’} makes integer from pointer without a cast [-Wint-conversion]
#    73 |     event.data.u64 = ocaml_value;
#       |                    ^

I would recommend to also do a release of gluon.