Add new subtypes to increase coverage of subtype application on issues and MRs

Why is this change being made?

To increase coverage of known gaps in the subtype taxonomy for Work Classification: https://about.gitlab.com/handbook/engineering/metrics/#work-type-classification

New subtypes

• ~"bug::functional": Functional defects resulting from feature changes
• ~"bug::ux": Unexpected and unintended behavior that is detrimental to the user experience.
• ~"maintenance::performance": Non-defect performance improvements such as load time for a specific page or component, or run time for a specific process.
• ~"maintenance::scalability": Modification to improve the scalability of GitLab that is not a user facing change or performance improvement. For example changing a column from INT to BIGINT.

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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