Clarify deprecation handling in the PhpUnitBridge

[xabbuh]

In https://symfony.com/doc/current/components/phpunit_bridge.html#mark-tests-as-legacy we say that there are four ways to declare a test as legacy:

There are four ways to mark a test as legacy:

• (Recommended) Add the @group legacy annotation to its class or method;
• Make its class name start with the Legacy prefix;
• Make its method name start with testLegacy instead of test;
• Make its data provider start with provideLegacy or getLegacy.

However, this is not true. Prefix the name of a data provider method with provideLegacy or getLegacy does not influence the error handling of the test method, but is only useful if your data provider actually triggers deprecations.