Dream alpha2: drop ppxlib constraints
Dream alpha2: drop ppxlib constraints

Now that ppxlib 0.21.1 and 0.22.1 are out, the constraints are no longer
necessary. They were meant to exclude 0.21.0 and 0.22.0, but it should
be enough to just rely on the .1 versions being installed, in most
cases.

ocaml/opam-repository,hannesm/opam-repository,fpottier/opam-repository,jonludlam/opam-repository,astrada/opam-repository,djs55/opam-repository,c-cube/opam-repository,yallop/opam-repository,jhwoodyatt/opam-repository,emillon/opam-repository,kkirstein/opam-repository,hannesm/opam-repository,lefessan/opam-repository,jhwoodyatt/opam-repository,Chris00/opam-repository,nberth/opam-repository,pirbo/opam-repository,codinuum/opam-repository,mjambon/opam-repository,mjambon/opam-repository,Chris00/opam-repository,Leonidas-from-XIV/opam-repository,djs55/opam-repository,andersfugmann/opam-repository,nberth/opam-repository,ocaml/opam-repository,seliopou/opam-repository,pqwy/opam-repository,kkirstein/opam-repository,astrada/opam-repository,pqwy/opam-repository,pirbo/opam-repository,toots/opam-repository,codinuum/opam-repository,yallop/opam-repository,toots/opam-repository,lefessan/opam-repository,fpottier/opam-repository,jonludlam/opam-repository,c-cube/opam-repository,abate/opam-repository,emillon/opam-repository,andersfugmann/opam-repository,Leonidas-from-XIV/opam-repository,seliopou/opam-repository