submodule: bump openthread from ffe2f52 to aa8dfe1

Bumps [openthread](https://github.com/openthread/openthread) from `ffe2f52` to `aa8dfe1`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@ffe2f52...aa8dfe1)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

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