fix(platform/gitlab): Strip unicode null characters from markdown

[nfm]

Attempting to post a comment with null characters in them fails with a 400 error. See https://github.com/renovatebot/renovate/discussions/19532 for more details.

Changes

Strip unicode null characters from GitLab markdown, as their presence prevents comments from being posted. bundler appears to generate some error messages that include them.

Context

https://github.com/renovatebot/renovate/discussions/19532

Documentation (please check one with an [x])

• [ ] I have updated the documentation, or
• [x] No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• [ ] Code inspection only, or
• [x] Newly added/modified unit tests, or
• [ ] No unit tests but ran on a real repository, or
• [ ] Both unit tests + ran on a real repository