Update approvers_select to use listbox

What does this MR do and why?

This MR updates the approvers_select to remove select2 and to use gl-collapsible-listbox instead.

• Update ee/app/assets/javascripts/approvals/components/approvers_select.vue
  • use GlCollapsibleListbox instead of the select2
  • use GlAvatarLabeled instead of the custom template (requires a small visual change as the avatar is updated to the closest GitLab UI size).
  • avoid using !this.isFeatureEnabled within the if (this.isFeatureEnabled) block in the fetchGroups method.
• Add proper styles to selects.scss in order to achieve the full width gl-collapsible-listbox (until gitlab-ui#2099 (closed) is done)
• Update related specs
• Fix some false positive specs:
  • user_edits_approval_rules_mr_spec.rb#L93 is only passing when the permit_all_shared_groups_for_approval feature flag is off as the logic is only set for the Api.groups endpoint and not the Api.projectGroups
  • user_sets_approvers_spec.rb#L26 was testing an absent functionality. It was indeed possible to add an author as an approver. I fixed it by adding an MR author user id into the list of skipped user ids
  • user_sets_approvers_spec.rb#L72 is only passing for the group other then the one project belongs to.

Screenshots or screen recordings

The select appears in 3 places:

1. Project -> Settings -> Merge requests

Before	After

2. Merge request -> Edit -> Approval rules

Before	After

3. Security & Compliance -> Policies -> New policy -> Scan result policy

Before	After

How to set up and validate locally

Visit the corresponding pages:

1. Project -> Settings -> Merge requests
2. Merge request -> Edit -> Approval rules
3. Security & Compliance -> Policies -> New policy -> Scan result policy

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.

Review progress:

• UX review
• test review
• backend developer review
• backend maintainer review
• frontend developer review
• frontend maintainer review

Related to #374115 (closed)