Support for PHP 8

[kdambekalns]

• Travis tests on PHP 8 since TASK: Test against PHP 8 #2015
• doctrine/dbal runs with PHP8 as of 2.12.0: [2.12] PHP 8 compatibility doctrine/dbal#4347
• doctrine/orm is on the way: PHP8 Support doctrine/orm#8303

[kdambekalns]

doctrine/commons is raised to 2.13.1 in #1880 and supports PHP 8 as of that version.

[kdambekalns]

doctrine/dbal is raised to 2.12 in #1880 and supports PHP 8 as of that version.

[kdambekalns]

The Doctrine dependencies need to be raised in Neos, too, once #1880 is merged.

[albe]

With #2287 existing code runs on PHP 8. However, to fully support PHP 8 we need to be able to deal with union types, constructor properties and annotations (at least for proper proxy building).

First step is maybe to add meta-info on union types to the reflection service.

Edit: See #2404

[albe]

PHP8 Attributes are now supported as of #2468 but #2404 is still WIP

[bwaidelich]

@albe the related PRs have been merged, can this one be closed?