Allow "ci skip" to be in any case

What does this MR do?

This MR makes the check for the [ci skip] tag in a commit message case insensitive and adds support for [skip ci].

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

I couldn't understand why some of my commits were being built, even though they contained "[CI Skip]".

What are the relevant issue numbers?

N/A

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together