Add 11.7 -> 11.8 update doc

What does this MR do?

This adds the 11.7 -> 11.8 update. It might be a bit early, but I thought it is better to add it early so don't forget to adjust it with: https://gitlab.com/gitlab-org/gitlab-ce/issues/54065

What are the relevant issue numbers?

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/54065

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer