[MM-62973][MM-63028][MM-63034][MM-63027] Various accessibility fixes around Browse Channels modal (#30922)

* [MM-62973] Fix order of Close button and other buttons in the header

* [MM-63028] Add aria-live status area when searching for channels

* [MM-63027] Add announcement that channel is joined

* [MM-63034] Add role=checkbox and reformat HTML to have correct custom checkbox code

* Update snapshots from generic_modal change

* PR feedback

* PR feedback

* PR feedback

e2e-tests/cypress/tests/integration/channels/channel/browse_public_channels_spec.ts

@@ -20,12 +20,12 @@ function verifyNoChannelToJoinMessage(isVisible) {
 function ensureHideJoinedCheckboxEnabled(shouldBeChecked) {
     cy.get('#hideJoinedPreferenceCheckbox').then(($checkbox) => {
         cy.wrap($checkbox).findByText('Hide Joined').should('be.visible');
-        cy.wrap($checkbox).find('button').invoke('attr', 'class').then(($classList) => {
+        cy.wrap($checkbox).find('div.get-app__checkbox').invoke('attr', 'class').then(($classList) => {
             if ($classList.split(' ').includes('checked') ^ shouldBeChecked) {
                 // We click on the button only when the XOR operands do not match
                 // e.g. checkbox is checked, but should not be checked; and vice-versa
                 cy.wrap($checkbox).click();
-                cy.wrap($checkbox).find('button').should(`${shouldBeChecked ? '' : 'not.'}have.class`, 'checked');
+                cy.wrap($checkbox).find('div.get-app__checkbox').should(`${shouldBeChecked ? '' : 'not.'}have.class`, 'checked');
             }
         });
     });