Search board issues by iteration cadence

What does this MR do and why?

Add the ability to search issue boards by iteration cadence.

The backend currently doesn't support negated filtering for iteration cadences nor wild cards (NONE, ANY) so this is just a basic filter by cadence.

Behind :iteration_cadences feature flag.

Screenshots or screen recordings

How to set up and validate locally

• Turn on :iteration_cadences feature flag,
• Load an issue board
• Test Iteration cadence filter option.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #354041 (closed)