Fix wrong data collector signature

[alcaeus]

When adding compatibility with Symfony 5 in #602, we missed that Symfony 3.4 was still supported. Supporting 3.4 and 5.0 at the same time is not possible due to a method signature change in the data collector.

This PR fixes this issue by removing support for Symfony 3.4, which will be released as 4.1.0. This has the following implications:

• 4.0.* will no longer be supported. Users currently using Symfony 3.4 alongside ODM 2.0 should upgrade to Symfony 4.3 or newer as soon as possible.
• 4.1.* will be the new supported release for ODM 2
• 3.6.* still supports Symfony 3.4 and ODM 1.3 and will continue to do so until the end of life of ODM 1 is reached.