Bump third_party/imgui/repo from 3c435c0 to 2860d7b (#37605)

Bumps [third_party/imgui/repo](https://github.com/ocornut/imgui) from `3c435c0` to `2860d7b`.
- [Release notes](https://github.com/ocornut/imgui/releases)
- [Commits](ocornut/imgui@3c435c0...2860d7b)

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

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