feat(elasticsearch): re-introduce v7 support

[darthf1]

Q	A
Branch?	main
Tickets	closes #6788
License	MIT
Doc PR	api-platform/docs#...

This PR re-introduces ES7 support, by simply introducing the v7 client namespaces.

[darthf1]

@soyuka i tried to add back ES v7 support, with as less changes as possible. I commented above the code whether it involves ES v7 or v8 and up, which should make maintenance a bit easier, also when ES 7 has been declared EOL. There were still some ES7 checks in the code, so i just added comments above them.

As for the phpstan errors; can I add them to the baseline? I get more errors locally than in CI, so I'm not sure if everything is setup correctly. In the phpstan.neon.dist keys like containerXmlPath and constantHassers are used, but those are phpstan v2 keys.

Behat tests fail, they give the following error:

In Validator.php line 61:
                                                                                                                                        
  Can not find a matching value for an argument `$client` of the method `ApiPlatform\Tests\Behat\ElasticsearchContext::__construct()`.  

I'm not a Behat user; do you maybe have some thoughts how to resolve this? I added a new behat job for ES v7, it looks like these are successful.