Update gitlab-markup to v1.8.1

What does this MR do and why?

Update gitlab-markdup gem to v1.8.1 which contains Fix compatibility with Busybox timeout(1) (gitlab-markup!37 - merged)

This is a low-risk change, the new syntax for the timeout option is valid and supported: https://man7.org/linux/man-pages/man1/timeout.1.html

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