fix(dropdowns): Rename section header, remove deprecated exports

What does this MR do?

This breaking change renames GlDropdownHeader to GlDropdownSectionHeader. This change is needed in order to not confuse the top level header of a dropdown that is available as header-text with a gl-dropdown-header. This also aligns GitLab UI with naming conventions in Figma.

There are four files that will need to be updated in the GitLab code base:

app/assets/javascripts/snippets/components/embed_dropdown.vue:
   6:   GlDropdownHeader,
  21:     GlDropdownHeader,

ee/app/assets/javascripts/analytics/cycle_analytics/components/stage_dropdown_filter.vue:
   4:   GlNewDropdownHeader as GlDropdownHeader,
  13:     GlDropdownHeader,

ee/app/assets/javascripts/analytics/shared/components/groups_dropdown_filter.vue:
   8:   GlNewDropdownHeader as GlDropdownHeader,
  24:     GlDropdownHeader,

ee/app/assets/javascripts/analytics/shared/components/projects_dropdown_filter.vue:
   8:   GlNewDropdownHeader as GlDropdownHeader,
  24:     GlDropdownHeader,

This MR also removes deprecated exports using GlNewDropdown. We will need to search and replace all the following in the GitLab code base and any other project importing GitLab UI:

• GlNewDropdownItem
• GlNewDropdownHeader
• GlNewDropdownDivider
• GlNewDropdownText
• GlNewDropdown

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.