Bump extern/amrex from 6d9c25b to b9d549b

Bumps [extern/amrex](https://github.com/AMReX-Codes/amrex) from `6d9c25b` to `b9d549b`.
- [Release notes](https://github.com/AMReX-Codes/amrex/releases)
- [Commits](AMReX-Codes/amrex@6d9c25b...b9d549b)

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

Signed-off-by: dependabot[bot] <[email protected]>