Add monthly day dropdown

What does this MR do and why?

Add monthly day dropdown

• add selection for monthly days
• add tests

EE: true

Plan

• Run for/duration input is being worked on in Create duration input and dropdown (!185026 - merged)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Upload a GitLab Ultimate license
2. Enable the feature flag via terminal or http://gdk.test:3443/rails/features

echo "Feature.enable(:scheduled_pipeline_execution_policies)" | rails c

3. Navigate to a project => Secure => Policies => New policy => Pipeline execution policy => Select "schedule a new"
4. Change the cadence to monthly
5. Verify updating the days of the month updates the yaml

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #505173 (closed)