chore: Adapt meta.yml to v8.20 & Add dedicated CI test using coqorg/base

[erikmd]

No description provided.

[erikmd]

@proux01 the CI is green with this new version of bignums' CI that doesn't require a docker-coq image with bignums

two questions:

• this PR is ready from my viewpoint, feel free to merge it or squash it tomorrow Thursday if you can

• next, before I can prepare an image of coqorg/coq:8.20-alpha with coq-bignums (continuously built from coq branch v8.20), is it OK that we release a new tag of bignums v9.0.0+coq8.20?
  or should we just add a version in extra-dev from the bignums branch v8.20 (which might be tricky w.r.t. the naming convention maybe)

[proux01]

Thanks @erikmd , yes feel free to go ahead with the tag, that's what we usually do for each Coq release.

[erikmd]

just FYI @proux01 I ported the new CI workflow to the master branch: 379748e