Remove deprecated from admin

[joshcanhelp]

Changes

Note to reviewers: Around 2/3s of the lines added are new tests. Most of the functionality "added" here existed in different methods.

• Move WP_Auth0_Settings_Section::init_menu() method to new wp_auth0_init_admin_menu() function
• Remove WP_Auth0_Settings_Section class
• Rename wpa0_admin_initial_settup style to wpa0_admin_initial_setup
• Move WP_Auth0_Admin::create_account_message() method to new wp_auth0_create_account_message() function
• Remove WP_Auth0_Admin::init() method
• Remove WP_Auth0_Admin::cant_connect_to_auth0() method
• Remove WP_Auth0_Admin_Generic::rule_validation() method
• Remove WP_Auth0_Admin_Generic::render_a0_switch() method
• Only load core WP media styles and scripts on settings page.

Testing

• This change adds unit test coverage
• This change has been tested on WP 5.3

Checklist

• All existing and new tests complete without errors
• All code quality tools/guidelines in the Contribution guide have been run/followed
• All active GitHub CI checks have passed