[MM-62989][MM-63039][MM-63033][MM-64174][MM-63040][MM-63032][MM-63001][MM-63006][MM-63037] Various accessibility fixes for the Create Channel modal (#30888)

* [MM-62989] Replace channel purpose input with Input component

* [MM-63039] Fix styles so that a11y--focused box shadow is applied to the private/public channel buttons

* [MM-63033] Add aria-describedby and role=alert to the URL input on create channel modal

* Fix lint

* [MM-63035] Change legend to label, fieldset to div in the Input component

* Fix i18n

* [MM-64174] Stop propagation of enter event while editing the new channel

* [MM-63037] Add role=img and an aria label to the error img icon

* Fix snap

* Revert "[MM-63035] Change legend to label, fieldset to div in the Input component"

This reverts commit e8516f3e6a266c77db2b1695a036db717041a9ef.

* [MM-63040] Remove tabindex=0 from GenericModal wrapper, fix issue with URLInput that I caused D:

* Fix snap

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/platform/components/src/generic_modal/__snapshots__/generic_modal.test.tsx.snap

@@ -34,7 +34,6 @@ exports[`GenericModal should match snapshot for base case 1`] = `
           >
             <div
               class="GenericModal__wrapper GenericModal__wrapper-enter-key-press-catcher"
-              tabindex="0"
             >
               <div
                 class="modal-header"

webapp/platform/components/src/generic_modal/generic_modal.tsx

@@ -115,7 +115,6 @@ export const GenericModal: React.FC<Props> = ({
     compassDesign,
     backdrop,
     backdropClassName,
-    tabIndex,
     children,
     autoFocusConfirmButton,
     headerInput,
@@ -288,7 +287,6 @@ export const GenericModal: React.FC<Props> = ({
             <div
                 ref={containerRef}
                 onKeyDown={onEnterKeyDown}
-                tabIndex={tabIndex || 0}
                 className='GenericModal__wrapper GenericModal__wrapper-enter-key-press-catcher'
             >
                 {showHeader && (