Release OCaml/Unikraft 1.1.0

[shym]

Changes

• Support for OCaml 5.4.0 and Unikraft 0.20.0
• Various fixes, in particular an issue where the OCaml cross compiler failed to build in an opam local switch

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This PR proposes also to align the 0.18.0 versions of the ocaml-unikraft-toolchain-* packages with the new versions, to make sure the same version number is used across all backend packages.

[shym]

Analysis of the Opam CI results:

1. Debian 11, OpenSuse 15.6: Unikraft 0.20.0 uses an attribute introduced in GCC 11 without first testing for it, reported upstream.
2. CentOS 10: conf-python-3 fails to install (repology seems to suggest it packages 3.12, so we might have to update our package).
3. Failures of the linter are due to the fact that it’s over-cautious and rejects virtual packages unless they enter one of its categories (such as conf-).
4. Alpine, CentOS 9, OpenSuse 15.6, OpenSuse Tumbleweed: the C cross compiler to arm64 is not packaged for those distributions, so this fails trying to install conf-aarch64-linux-gnu-gcc.

For 1., I wonder whether we should try to exclude older GCCs, but I don’t know what’s the best way to do so.
For 3. and 4., they are already present in OCaml/Unikraft 1.0.0.

[jmid]

Thanks - also for the CI summary! 🙏

For 4., we might consider adding x-ci-accept-failures entries to silence the red CI lights here on the opam-repo, while still allowing folks on those platforms to install as long as they have a locally installed cross-compiler?