update macOS screenshots for BigSur

backups/backuppc_osx.markdown

mail/how_to_use_email_with_apple_mail.markdown

media/osx/802_1x_2.png

media/osx/apple_mail_settings_1.png

media/osx/apple_mail_settings_2.png

media/osx/apple_mail_settings_3.png

media/osx/apple_mail_settings_4.png

media/osx/apple_mail_settings_5.png

media/osx/configure_apple_mail_1.png

media/osx/configure_apple_mail_2.png

media/osx/configure_apple_mail_3.png

media/osx/configure_apple_mail_4.png

media/osx/configure_apple_mail_5.png

media/osx/configure_apple_mail_6.png

media/osx/enable_remote_login.png

media/osx/enable_remote_login_macosx.png

media/osx/filevault_1.png

media/osx/filevault_2.png

media/osx/filevault_3.png

media/osx/filevault_4.png