Add upper bounds on `ocaml-compiler-libs` for boltzgen.0.9 and ppxx.2.5.0

[shonfeder]

@mseri reported that these two packages are failing in CI revdeps tests due to breaking changes in ocaml-compiler-libs. This was caught but the following revdeps tests:

• https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/fee7f3fbdd0250a19e35720bb55a202b18069834/variant/compilers,5.3,base.v0.17.3,revdeps,boltzgen.0.9
• https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/fee7f3fbdd0250a19e35720bb55a202b18069834/variant/compilers,5.3,base.v0.17.3,revdeps,ppxx.2.5.0

An example of what needs to be done (but for a different package) can be found at https://github.com/ocaml/opam-repository/pull/28020.

Documentation for adding upper bounds for breaking revdeps can be found at https://github.com/ocaml/opam-repository/wiki/Revdeps