Manifest: fixes that were needed to release Octez 15.0 on opam

Context

Trying to opam install Octez results in issues opam may select older or newer versions of some packages, which may not be compatible. Those incompatibilities have been identified in the CI of the Octez opam release (https://github.com/ocaml/opam-repository/pull/22482). This MR backports those fixes to master.

One other issue was that the linter failed to validate a synopsis. I added a check in the manifest and fixed the culprit.

Manually testing the MR

Checklist

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