Don't mention assignees in the 'MR is ready' message

What does this MR do and why?

As reported in Slack, pinging the assignee in addition to actually assigning them creates a duplicate ToDo item, which is mostly noise.

By putting the mentioned usernames in backticks we avoid the double ToDo while still addressing the note to the assigned reviewers.

For what it's worth, we already use this strategy in the NewPipelineOnApproval processor.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review