Update iteration quick action to accept cadence reference

Release notes

The /iteration quick action now accepts a cadence reference with --current or --next arguments. If your group has a single iteration cadence, you can quickly assign an issue to the current or next iteration via the /iteration --current|next. If your group contains many iteration cadences, you can specify the desired cadence in the quick action by referencing the cadence name or ID. For example, /iteration [cadence:"<insert cadence name>"|<cadence ID>] --next|current.

https://docs.gitlab.com/ee/user/project/quick_actions.html#issues-merge-requests-and-epics

Proposal

Referencing

Introduce a new reference scheme for Iterations Cadence

• [cadence:<CADENCE_ID>]
• [cadence:<CADENCE_NAME>] This reference will fail (and be left as a plain text) if no accessible cadence matches the given name.

Quick actions

• /iteration [cadence:<CADENCE_ID>] [--current (default)|--next]
• /iteration [cadence:"<CADENCE_NAME>"] [--current (default)|--next]

Acceptance

• The format uses our new extensible reference syntax [cadence: ...]
• Can select Current from within a cadence, which assigns the issue to the current iteration within a cadence.
• Can select Next from within a cadence, which assigns the issue to the next iteration within a cadence.

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