chore: Avoid result < 1.5 when installing, as this breaks on 4.08+

[Leonidas-from-XIV]

Detected as part of #10300, the alternate solution in ocaml/opam-repository#25603 is unlikely to go forward at this point so this implements the policy of opam-repository and adds the conflict on our side.

[anmonteiro]

Is this needed after #10261 ?

[Leonidas-from-XIV]

This is necessary because of #10261. Before we required result.1.5 and everything worked, but after removing it (which is a good thing!) package solutions could going forward pick result < 1.5 (in the case of the CI failure linked it was result.1.0) which defines a result type that is not compatible with that one in 4.08+, so when dune-rpc-lwt would use the 4.08-builtin result and another package would use result.1.0-provided result they wouldn't be compatible.

My first thought was to restrict result to only have result.1.5 on 4.08+ so I posted ocaml/opam-repository#25603 but as you can read in the issues linked it wasn't the first time that this has bit someone. For now packages are supposed to conflict with result until compatibility with versions older than 4.08 is dropped from opam-repository.

In practice this mostly affects the low-version check in opam-repository, because users would get result.1.5 as that is the newest version and a number of other packages already conflict on result < 1.5.

[anmonteiro]

understood, thanks for the details!