Move OnCall Schedules timezone to the shared Timezone Dropdown

What does this MR do and why?

1. Migrates custom GlDropdown in creating oncall schedule modal, to a shared TimezoneDropdown. There is a slight difference in formatting ([] instead of () for UTM) - I decided to keep it as is for consistency across application (see CI/CD > Schedules > New Schedule for reference).
2. Adds a dropdownClass property to customise shared dropdown. In case of oncall form, extra margin did not allow to display validation message directly under the box. Hence the addition and reset to 0px.
3. Adds a unit test to TimezoneDropdown for marking selected timezone.

Screenshots or screen recordings

before	after

How to set up and validate locally

Prerequisite:

• This is a premium feature
• You must have at least the Maintainer role.

To open create an on-call schedule modal:

1. On the top bar, select Main menu > Projects and find your project.
2. On the left sidebar, select Monitor > On-call Schedules.
3. Select Add a schedule.
4. Enter the schedule’s name and description and select a time zone.

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 #378631 (closed)

ee/spec/features/oncall_schedules/user_creates_schedule_spec.rb

@@ -28,7 +28,7 @@
       fill_in 'schedule-description', with: 'Test schedule description'
 
       click_button 'Select timezone'
-      click_button '(UTC -12:00) -12 International Date Line West'
+      click_button '[UTC - 12] International Date Line West'
       click_button 'Add schedule'
 
       wait_for_all_requests