From the mathcomp 1.9.0 release notes:

> removed Coq prelude hints plus_n_O plus_n_Sm mult_n_O mult_n_Sm, to
> improve robustness of by ...; scripts may need to invoke addn0,
> addnS, muln0 or mulnS explicitly where these hints were used
> accidentally.

=> This patch makes these required fixes in Prosa.

While at it, turn on CI for coq:dev and Coq 8.9 with two versions of ssreflect.