Merge `User#GetDisplayName` into `User#DisplayName`

[delvh]

Feature Description

As seen for example in #31687, we currently have two different methods to display a user name, and both doing similar things.
I don't see a reason why we should keep both.
I think keeping DisplayName and merging the body of GetDisplayName would make the most sense.

Or is there any reason why we need both methods, where we once ignore the [ui].DEFAULT_SHOW_FULL_NAME setting?

Screenshots

No response

[wolfogre]

I agree. I believe it's just historical baggage.