Bump extern/amrex from cf8d9f8 to de7c618 (#354)

Bumps [extern/amrex](https://github.com/AMReX-Codes/amrex) from `cf8d9f8` to `de7c618`.
- [Release notes](https://github.com/AMReX-Codes/amrex/releases)
- [Commits](AMReX-Codes/amrex@cf8d9f8...de7c618)

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

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