Update Mirage dependencies

[yomimono]

This makes pgx_lwt_mirage build with modern Mirage dependencies. It builds on #116 .

[gtrak]

@yomimono are you done with all the changes you plan to do here? We're ready to check if we can integrate it. Are the Ci tests failing because of your changes?

#!/bin/bash -eo pipefail
cd unikernel && opam exec -- mirage configure -t spt

                                  (cd _build/default && /home/opam/.opam/4.11/bin/ocamlc.opt -w @1..3@5..28@30..39@43@46..47@49..57@61..62-40 -strict-sequence -strict-formats -short-paths -keep-locs -warn-error -A -g -bin-annot -I .config.eobjs/byte -I /home/opam/.opam/4.11/lib/astring -I /home/opam/.opam/4.11/lib/bos -I /home/opam/.opam/4.11/lib/cmdliner -I /home/opam/.opam/4.11/lib/domain-name -I /home/opam/.opam/4.11/lib/fmt -I /home/opam/.opam/4.11/lib/fpath -I /home/opam/.opam/4.11/lib/functoria -I /home/opam/.opam/4.11/lib/functoria-runtime -I /home/opam/.opam/4.11/lib/functoria/app -I /home/opam/.opam/4.11/lib/ipaddr -I /home/opam/.opam/4.11/lib/logs -I /home/opam/.opam/4.11/lib/lwt -I /home/opam/.opam/4.11/lib/macaddr -I /home/opam/.opam/4.11/lib/mirage -I /home/opam/.opam/4.11/lib/mirage-runtime -I /home/opam/.opam/4.11/lib/ocamlgraph -I /home/opam/.opam/4.11/lib/ptime -I /home/opam/.opam/4.11/lib/ptime/clock/os -I /home/opam/.opam/4.11/lib/result -I /home/opam/.opam/4.11/lib/rresult -I /home/opam/.opam/4.11/lib/seq -I /home/opam/.opam/4.11/lib/stdlib-shims -no-alias-deps -opaque -o .config.eobjs/byte/config.cmo -c -impl config.ml)
File "config.ml", line 61, characters 8-13:
61 |       $ stack
             ^^^^^
Error: This expression has type stackv4v6 impl
       but an expression was expected of type stackv4 impl
       Type stackv4v6 is not compatible with type stackv4 
mirage: unknown option '-t'.     

[yomimono]

Yes, I hadn't realized there was a sample unikernel, which I need to make a few more edits to. I'll at-mention you once the tests are passing.

[yomimono]

@gtrak Passing now and ready for review whenever someone has the time. Thanks!

[gtrak]

Ok, this looks good. I'll land it, and I guess we can cut a release or something later.