Update new iterations form to match pajamas specs

What does this MR do and why?

• Update form to use GlDatepicker
• Update form to show errors on required fields, title & start date
• Use GlFormGroup where required

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Navigate to /groups/gitlab-org/-/iterations/new
2. Click Create iteration without filling out any fields, this will show error message and highlight the required fields
3. Enter input in the required fields, and click Create iteration, this should Submit the form successfully

Next steps

Update design-ui datepicker to accept input labels for better Capybara testing