Add support for version upgrades (#4160)

Add support for version upgrades (#4160)

* Add WCF::AVAILABLE_UPGRADE_VERSION

* Expose the available upgrade version to packageList.tpl

* Show notice in packageList if an upgrade is available

* Add upgrade override support to PackageUpdateServer

* Show notice about enabled upgrade override in packageList and packageUpdateServerList

* Add PackageEnableUpgradeOverrideForm

* Support disabling the upgrade override if there are issues

* Add version checks from update_com.woltlab.wcf_5.4_checkSystemRequirements.php to PackageEnableUpgradeOverrideForm

* Add language items for WoltLab Suite upgrades

* Clear package lists when toggling the upgrade override

* Add requested-woltlab-suite-version header to package list requests

* Improved the messages and restrict the upgrade to the maintenance mode

* Restrict the experimental upgrade to the maintenance mode

* Removed the upgrade notice from the package server list

Co-authored-by: Alexander Ebert <ebert@woltlab.com>