Bump extern/amrex from 9c256b1 to cf8d9f8 (#340)

Bumps [extern/amrex](https://github.com/AMReX-Codes/amrex) from `9c256b1` to `cf8d9f8`.
- [Release notes](https://github.com/AMReX-Codes/amrex/releases)
- [Commits](AMReX-Codes/amrex@9c256b1...cf8d9f8)

---
updated-dependencies:
- dependency-name: extern/amrex
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>