Use show() method for opening datepicker

What does this MR do and why?

This MR is an integation MR for gitlab-ui!3109 (merged) updating usage of .calendar.show()

Screenshots or screen recordings

N/A

How to set up and validate locally

🍏 Green pipeline means it's ok 🚀

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.