chore: rename barretenberg_blackbox_solver to bn254_blackbox_solver

[TomAFrench]

Description

Problem*

Resolves

Summary*

This PR renames barretenberg_blackbox_solver to bn254_blackbox_solver and marks it as not deprecated.

This signifies that these blackbox solvers should work against all backends targetting the bn254 curve and not just barretenberg. That we're using barretenberg to solve them is then just an implementation detail.

We've decided to retain the BlackBoxFunctionSolver trait to allow these solvers to be swapped out based on the FieldElement implementation and so the new Bn254BlackBoxSolver is not deprecated.

Additional Context

Now that this solver is tied to a curve rather than a particular backend, it's now proper to integrate it into the compiler such that we can solve black box functions such as that which caused #3865

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.