ci: bump SimHub build dependency to 9.12.6

[github-actions]

Bumps the pinned SimHub build dependency 9.11.22 -> 9.12.6.

• Release notes: https://github.com/SHWotever/SimHub/releases/tag/9.12.6
• Pre-merge validation (build + tests on main against SimHub 9.12.6): passed — https://github.com/kelchm/FanaBridge/actions/runs/34833493650

Note: PRs opened by GITHUB_TOKEN don't trigger CI; validation above ran pre-merge
against main. Close and reopen this PR (or push to it) to run normal CI checks.

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