build(deps): bump Externals/imgui from c807192 to 6ca1556

Bumps [Externals/imgui](https://github.com/ocornut/imgui) from `c807192` to `6ca1556`.
- [Release notes](https://github.com/ocornut/imgui/releases)
- [Commits](ocornut/imgui@c807192...6ca1556)

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

Signed-off-by: dependabot[bot] <[email protected]>