Set an upper bound of dream-httpaf for dream.1.0.0~alpha5-7

[jmid]

This PR sets upper bounds for dream-httpaf in dream.1.0.0~alpha5-7
as using dream-httpaf.1.0.0~alpha4 causes a compilation failure.
We've previously set lower bounds for it in #27034.

This was spotted in #28267
https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/d0ec749105986928c2134f9046e9b9382af0aafe
causing

[ERROR] The compilation of dream.1.0.0~alpha7 failed at "dune build -p dream -j 71".

#=== ERROR while compiling dream.1.0.0~alpha7 =================================#
# context              2.3.0 | linux/x86_64 | ocaml-base-compiler.4.14.2 | file:///home/opam/opam-repository
# path                 ~/.opam/4.14/.opam-switch/build/dream.1.0.0~alpha7
# command              ~/.opam/opam-init/hooks/sandbox.sh build dune build -p dream -j 71
# exit-code            1
# env-file             ~/.opam/log/dream-7-00e69c.env
# output-file          ~/.opam/log/dream-7-00e69c.out
### output ###
# File "src/http/dune", line 21, characters 2-32:
# 21 |   dream-httpaf.dream-websocketaf
#        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# Error: Library "dream-httpaf.dream-websocketaf" not found.
# -> required by library "dream.http" in _build/default/src/http
# -> required by _build/default/META.dream
# -> required by _build/install/default/lib/dream/META
# -> required by _build/default/dream.install
# -> required by alias install

To clear up numbering: dream-httpaf.1.0.0~alpha4 was released with dream.1.0.0~alpha8 in 0750dde.
The opam file for dream-httpaf explicitly says "This package does not have a stable API." and the release https://github.com/aantron/dream/releases/tag/1.0.0-alpha8 mentions
Replace vendored http/af stack with depending on httpun from opam

Hopefully this will eliminate a few more CI red lights... 🤞

[jmid]

CI summary:

• 3 opam.2.0 time outs
• 2 curl failures on revdep asn1-combinators.0.2.6
• 6 failures to build conf-postgresql under macOS homebrew

None of these were caused by setting a dream-httpaf upper bound. As such, this should be good to go.

[mseri]

Thanks a lot