Bump third_party/imgui/repo from 96839b4 to f50ddc4 (#31917)

Bumps [third_party/imgui/repo](https://github.com/ocornut/imgui) from `96839b4` to `f50ddc4`.
- [Release notes](https://github.com/ocornut/imgui/releases)
- [Commits](ocornut/imgui@96839b4...f50ddc4)

---
updated-dependencies:
- dependency-name: third_party/imgui/repo
  dependency-type: direct:production
...

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