Update CompPoly toolchain to Lean 4.23 to support integration with hax Lean backend

[klausnat]

Request: consider updating CompPoly’s toolchain to leanprover/lean4:v4.23.0 (or a compatible 4.23.* version).

• hax Lean backend is pinned to Lean 4.23 since PR #1696.
• CompPoly currently pins leanprover/lean4:v4.22.0-rc2.

As a result, it is currently not possible to:

• build the hax Lean prelude (extraction backend) in a project that uses CompPoly, or
• directly connect Rust code extracted via hax to CompPoly specifications in the same Lean project.

Updating CompPoly to Lean 4.23 would enable the end-to-end pipeline:
Rust → hax → CompPoly spec → proofs (without splitting across incompatible Lean versions).

[dhsorens]

taken care of in #46