Bump third_party/imgui/repo from 1ab63d9 to 454f36d (#30435)

Bumps [third_party/imgui/repo](https://github.com/ocornut/imgui) from `1ab63d9` to `454f36d`.
- [Release notes](https://github.com/ocornut/imgui/releases)
- [Commits](ocornut/imgui@1ab63d9...454f36d)

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

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