Group by cadence in iteration dropdown widgets [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Iteration cadences &5077 are being introduced behind the feature flag :iteration_cadence which is off by default.

This MR updates the iteration dropdown widgets to group by cadence in the:

• Issue sidebar
• Group boards drawer
• Project boards drawer
• Group bulk edit sidebar
• Project bulk edit sidebar

Implementation notes:

• A new separate iteration dropdown component IterationSidebarDropdownWidget was created which reuses SidebarDropdownWidget. This is so we don't have iteration-only logic in the generic SidebarDropdownWidget component.
• The :iteration_cadences feature flag will be removed after the release, so the conditionals were added in a way that's easy to delete.
• The feature flag yml file iteration_cadences.yml was moved from EE to CE because we're using the feature flag in CE controllers

Issue: #293925 (closed)

Screenshots (strongly suggested)

	:iteration_cadences FF off	:iteration_cadences FF on
Issue sidebar h5bp/html5-boilerplate/-/issues/1	off-issue	on-issue
Group board groups/h5bp/-/boards	off-group-board	on-group-board
Project board h5bp/html5-boilerplate/-/boards/3	off-project-board	on-project-board
Group bulk edit groups/h5bp/-/issues	off-group-bulk	on-group-bulk
Project bulk edit h5bp/html5-boilerplate/-/issues	off-project-bulk	on-project-bulk

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #293925 (closed)