submodule: bump openthread from 19203d3 to 2591b58 (#939)

Bumps [openthread](https://github.com/openthread/openthread) from `19203d3` to `2591b58`.
- [Commits](openthread/openthread@19203d3...2591b58)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 2591b58f3c0a0f97a1ad98f187a203970b7a8f1c
  dependency-type: direct:production
...

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

Author: dependabot[bot]