Document and lint that trailers are case-sensitive

What does this MR do?

The changelog API treats Git trailers case-sensitively in various places. We also do the same in templates when we use trailers such as EE and MR. Supporting all this in a case-insensitive manner is not feasible at this time.

Instead, we now document that trailers must match exactly what value is set in the trailer setting. In addition, Danger now produces an error when a changelog trailer is present but uses the wrong casing (e.g. changelog instead of Changelog).

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.