Upgrade setup-ocaml to v3.9.0

[cknitt]

Summary

• upgrade ocaml/setup-ocaml from v3.8.0 to v3.9.0, enabling opam 2.6.0
• update the OPAM cache key and reported setup version
• remove the explicit HTTP opam repository workaround, since v3.9.0 now uses it by default

[pkg-pr-new]

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commit: d7ea117