Update "upgrading" guide for Symfony5

[weaverryan]

This page: https://symfony.com/doc/current/setup/upgrade_major.html

• To be more realistic, we should change all "4" versions to "5"
• Change the instructions about updating symfony/symfony in composer.json to updating all your symfony/* packages
• I believe the user will also need to update the extra.symfony.require key in composer.json from (for example) 4.4.* to 5.0.*. This IS described correctly in the "upgrade minor" doc: https://symfony.com/doc/current/setup/upgrade_minor.html
• Remove the section on the bottom about updating to the Flex directory structure