Update hamlit to fix ruby 2.5 incompatibilities, fixes #42045

What does this MR do?

This updates hamlit to 2.8.8 (specifying a minimal version 2.8.5. This removes the obvious breakage from Gitlab when using ruby 2.5.

This is based on MR !19006 (closed), with the suggested changes included, to get this in for Gitlab 11.1.

Are there points in the code the reviewer needs to double check?

Hamlit 2.7.0 stopped filtering content in the plain filter (https://github.com/k0kubun/hamlit/pull/87). AFAICT this shouldn't affect Gitlab, but should be double checked.

Why was this MR needed?

Ruby 2.5 support.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #42045 (closed) Closes !19006 (closed)