Bump Submodules/amrex from 9751217 to 728eec4 (#488)

Bumps [Submodules/amrex](https://github.com/AMReX-Codes/amrex) from `9751217` to `728eec4`.
- [Release notes](https://github.com/AMReX-Codes/amrex/releases)
- [Commits](AMReX-Codes/amrex@9751217...728eec4)

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

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