submodule: bump openthread from 7761b81 to 7a2983b

Bumps [openthread](https://github.com/openthread/openthread) from `7761b81` to `7a2983b`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@7761b81...7a2983b)

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

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