fix: Updating slack channel name and fix emoji reference

Why is this change being made?

This MR fixes small issues observed during onboarding:

• Update the display name of the slack channel for internal colloboration/questions to the current name. The link works fine but the display name is wrong in this handbook page and changing it here should help avoid confusion about what channel should be used.
• Slack emoji style references were used e.g. :x: but that does not render an icon outside of slack so I've replaced it with the equivalent Unicode emoji characters so that it displays with the intended imagery on all systems that support emoji.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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