[tz] Use GST abbreviation for Asia/Dubai, Asia/Muscat

[Mohamed Akram]

> "GST" was my invention

Well now I feel silly. That’s true, it’s not often used (at least here in the UAE). Thank you for the clarification.

> On Jul 13, 2017, at 1:40 AM, Paul Eggert <eggert at cs.ucla.edu> wrote:
> 
> "GST" was my invention, and as I far as I know it is not typically used in English-language sources (other than those derived from tzdata). This is why it was removed in release 2017a, along with several other similar abbreviations. The idea is that tzdata should reflect common practice rather than prescribe it.