dune: bump to 3.8 · math-comp/multinomials@c344d24

The following actions uses node12 which is deprecated and will be forced to run on node16: actions/checkout@v2. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

card_mesymlmnm is declared opaque (Qed) but this is not fully

mesymlmE is declared opaque (Qed) but this is not fully respected

mesymlm_max is declared opaque (Qed) but this is not fully respected

XE is declared opaque (Qed) but this is not fully respected inside

inj_widen is declared opaque (Qed) but this is not fully respected

inj_swiden is declared opaque (Qed) but this is not fully respected

inj_mDswiden is declared opaque (Qed) but this is not fully

disjoint_S is declared opaque (Qed) but this is not fully respected

union_S is declared opaque (Qed) but this is not fully respected

mlead_XS is declared opaque (Qed) but this is not fully respected

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