Replace elimtype False by exfalso

[yforster]

@SkySkimmer this should fix Coq master again

[yforster]

I accidentally changed the CI to use coq-8.18 in this merge, that's why we didn't catch the incompatibility with Coq master -- now that's fixed as well

[JasonGross]

The opam files also need to be updated, eg,
https://github.com/MetaCoq/metacoq/blob/adc980ccf69be29be24db632de9ac8321ad54d44/coq-metacoq-utils.opam#L32
needs to permit dev

[yforster]

Thanks @JasonGross!

[yforster]

Still erroring in CI, but I think this error we've had before my merge as well? Good to merge into main @SkySkimmer?

[SkySkimmer]

I don't know why you're asking me tbh

[JasonGross]

I think this should be merged. I expect MetaCoq to still fail on Coq master with this universe error on vos mode, which I think is possibly a regression Coq-side. MetaCoq
CI side this can be fixed by disabling vos for whichever component is failing, which I seem to recall having to do for some part of quotation already.

[JasonGross]

Ah, the change I had already made was
https://github.com/MetaCoq/metacoq/blob/adc980ccf69be29be24db632de9ac8321ad54d44/Makefile#L103
in
https://github.com/MetaCoq/metacoq/blob/adc980ccf69be29be24db632de9ac8321ad54d44/Makefile#L84-L106