WordPress required version update

Hi !

Please note that BuddyPress 11.0.0 will require WordPress >= 5.7.

To learn more about this change: https://buddypress.trac.wordpress.org/ticket/8709

To learn more about how we decide to bump our WordPress required version: https://codex.buddypress.org/getting-started/wordpress-version-compatibility/

#11-0-0, #wp-requirements