[MA-7]: Refactored the post priority menu and fixed the keyboard navigation issue in the menu.

[ayush-chauhan233]

Summary

• Refactored the post-priority menu, removed the deprecated code, and fixed the keyboard navigation issue.
• Updated the menu component to accept a menu header as well as a menu footer.
• Updated the menu item role to menuitemcheckbox and menuitemradio and added appropriate roles to the menu.

Steps to reproduce

• Navigate to the Message priority button and select it.
• Notice that after selecting the Message priority button, the keyboard focus does not move on the dialog and also does not return to the triggered element.

Expected behavior

• As the menu opens it should auto-focus on the first menu item as well as all the other menu items should be keyboard accessible.

Ticket Link

Jira https://mattermost.atlassian.net/browse/MM-61570

Screenshots

Before	After

Release Note

- A minor change in the message priority checkbox menu item, the description width is smaller than the previous version.