Breaking: #81787 - Drop EXT:func

Description

The extension func that added the "Web->Functions" backend module has been dropped from core.

The extension is available in the TER and an install tool upgrade wizard is in place to download and load the extension as compatibility layer for other extensions that still rely on it by adding own sub modules to the module.

Extensions that need func should already have a dependency in ext_emconf.php similar to this:

'constraints' => [
    ...
    'depends' => [
        'func' => 9.0.0-9.0.99',
    ],
],

The version constraint depends on which core version the extension supports, the func extension did not change much between core v8 and its extraction to TER in v9.