Provide initial compatibility with PHP 8

[alcaeus]

Q	A
Type	improvement
BC Break	no
Fixed issues	Fixes #2222.

Summary

This PR makes the code base compatible with PHP 8. A few caveats:

• The match functionality in aggregation builder requires changing as match is now a reserved keyword. This PR includes changes from Rename Match classes for PHP 8 compatibility #2237.
• Tests are run on PHP 8, but we need to remove a few dev depdendencies as they block installing a PHP 8 compatible PHPUnit version.
• Until ocramius/proxy-manager is compatible with PHP 8 (see Allow PHP 8 Ocramius/ProxyManager#628) we need to run composer install on PHP 8 with --ignore-platform-req=php.
• Proxies and custom collections are not yet fully compatible with PHP 8 features such as union types. The former will be handled by ProxyManager but requires tests, while the latter will have to be taken care of in a separate pull request.

[jcaillot]

Thank you!

[alcaeus]

Note: I'll hold off on merging this until I've finished migrating the builds to GitHub actions. Can't rely on travis-CI anymore.

[garak]

Any news on this? I see that CI is running on github actions already

[alcaeus]

I lost track of this but will revisit soon. Thanks for the reminder 👍

[malarzm]

I've opened #2258 to track persistent collection changes

[malarzm]

I have merged 2.1.x branch into the master recently so you'll need to rebase your branch.

[alcaeus]

@malarzm I waited long enough to take care of proxies as well: we can now generate proxies with 7.4 language features on PHP 8 by switching to friendsofphp/proxy-manager-lts. Proxy support for PHP 8 features is still outstanding pending an upstream fix.

[malarzm]

Thanks for this!

[Slamdunk]

@malarzm can this PR be tagged so we can update to PHP 8?

[malarzm]

We'll try to have a release in January :)

[Slamdunk]

No hurry, I'm happy just with a plan 👍