Fix the commit linter to check that the first actual letter is capitalized

What does this MR do?

Prior to this change, Danger would complain that [API] The commit message is capitalized because the first char tested was [ with is the same when downcased. The linter now ignore any [...] prefix, and checks the first letter of the actual message instead (e.g. A in this example).

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Closes #208555 (closed).