Deprecate support for annotations

[garak]

This bundle currently supports both attributes and annotations, the latter mostly for BC purposes.
But attributes was introduced a while ago, with PHP 8.0, and now it's time to deprecate annotations.

Here's a list of tasks:

• convert all the tests code to use attributes instead of annotations
• remove attribute-related code examples in the documentation, mentioning that annotations are supported in old versions' docs
• add a deprecation message in VichUploaderExtension
• remove requirement for doctrine/annotations in our build scripts
• change mention of annotations in known_issues.md to explain it's still supported, but deprecated
• remove suggestion for doctrine/annotations in composer.json

PRs are welcome, even if meant to fix only a single point