CSAF-2.1: Adapt Mandatory Test 6.1.21

[DevPhilB]

In secvisogram #772 was reported that the revision history validation shows wrong entries.
We have decided that only the first error should be shown as validation result.

[peinjoh]

At csaf-rs, we have also been over how to implement 6.1.21 a few times now. We came up with this solution:
https://github.com/csaf-rs/csaf/blob/main/csaf-rs/src/validations/test_6_1_21.rs

If you decide to move away from only reporting the first error in the future, I hope this will save you some dev-hours 🤞