Remove support for D9

Problem/Motivation

At time of writing, all versions of 9 seem to be deprecated.
Due to the newly added Drupal 11 support, the following commit gives an error which breaks the working of the module for D9.x websites:
https://git.drupalcode.org/project/fac/-/commit/aec9e6f0835d766338e42b83...

Steps to reproduce

- Visit the homepage. In the page search, look for the "jsonFilesPath", which should look like "/sites/default/files/fac-json/general/en/{ID}".
- Visit the URL "{homepage}/{jsonFilesPath}/a.json", which doesn't return a json output, but gives the following backend error:
```Message Error: Class "Drupal\Core\File\FileExists" not found in Drupal\fac\Controller\FacController->generateJson() (line 172 of /web/modules/contrib/fac/src/Controller/FacController.php)```

Proposed resolution

The attached patch can be used.

Remaining tasks

Remove support for Drupal 9 from 2.x branch.

User interface changes

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API changes

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Data model changes

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