Replace DeprecatedModal with GlModal for boards

What does this MR do?

Refactor modal in app/assets/javascripts/boards/components/board_form.vue to use gitlab-ui GlModal.

Screenshots (strongly suggested)

Button	Modal before	Modal after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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 #229684 (closed)

ee/spec/features/boards/scoped_issue_board_spec.rb

@@ -297,6 +297,9 @@
           visit project_boards_path(project)
 
           update_board_label(label_title)
+
+          wait_for_all_requests
+
           update_board_label(label_2_title)
 
           expect(page).to have_css('.js-visual-token')
@@ -455,7 +458,7 @@
     it "doesn't show the input when creating a board" do
       click_on_create_new_board
 
-      page.within '.js-boards-selector' do
+      page.within '.js-board-config-modal' do
         # To make sure the form is shown
         expect(page).to have_field('board-new-name')
 
@@ -469,14 +472,14 @@
   end
 
   def expect_dot_highlight(button_title)
-    button = first('.filter-dropdown-container .btn.btn-inverted')
+    button = first('.filter-dropdown-container .btn.gl-button')
     expect(button.text).to include(button_title)
     expect(button[:class]).to include('dot-highlight')
     expect(button['title']).to include('This board\'s scope is reduced')
   end
 
   def expect_no_dot_highlight(button_title)
-    button = first('.filter-dropdown-container .btn.btn-inverted')
+    button = first('.filter-dropdown-container .btn.gl-button')
     expect(button.text).to include(button_title)
     expect(button[:class]).not_to include('dot-highlight')
     expect(button['title']).not_to include('This board\'s scope is reduced')