sagemathgh-35848: upgrade to flint3
    
Upgrade to flint3.

Current Sage versions are not compatible with flint ≥ 3, and, though the
diff is not huge, there are enough changes that versions including this
PR will be incompatible with flint < 3.

Fixes sagemath#20003.
Closes sagemath#35993 as no longer relevant.

Related PRs in upstream projects:
* Singular/Singular#1177
* flintlib/flint#1408
* flintlib/flint#1489
* flintlib/flint#1492
* flintlib/flint#1611
* algebraic-solving/msolve#76
* flatsurf/e-antic#264

Additional changes still needed for optional packages to work:
* sagemath#36677
* upgrade e-antic
* possibly more

Planned follow-ups:
* sagemath#36449
* sagemath#36433
    
URL: sagemath#35848
Reported by: Marc Mezzarobba
Reviewer(s): Vincent Delecroix

Author: Release Manager