build(deps): bump Externals/imgui from be2cfc2 to 913a3c6

Bumps [Externals/imgui](https://github.com/ocornut/imgui) from `be2cfc2` to `913a3c6`.
- [Release notes](https://github.com/ocornut/imgui/releases)
- [Commits](ocornut/imgui@be2cfc2...913a3c6)

---
updated-dependencies:
- dependency-name: Externals/imgui
  dependency-version: 913a3c60561bb07e8fd410ec7d4a8f6f485defd6
  dependency-type: direct:production
...

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Author: dependabot[bot]