submodule: bump openthread from 555454c to 7d488bc

Bumps [openthread](https://github.com/openthread/openthread) from `555454c` to `7d488bc`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@555454c...7d488bc)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

openthread

@@ -0,0 +1 @@
+7d488bcf244eca8944b0f07e33a97690d5a2798e