moment to dayjs, how to print UTC timezone flag?

**Describe the bug**

I'm coming from `moment`, switching to `dayjs` and struggle with the display of UTC timestamps. I want the current timestamp in UTC format, including the timezone.
in `moment`, I called:
```
const moment = require('moment')
moment.utc().format('HH:mm:ss z')
'14:21:49 UTC'
```

with `dayjs`, I call:
```
const dayjs = require('dayjs')
const utc = require('dayjs/plugin/utc.js')
const timezone = require('dayjs/plugin/timezone.js')
const advancedFormat = require('dayjs/plugin/advancedFormat.js')
dayjs.extend(utc)
dayjs.extend(timezone)
dayjs.extend(advancedFormat)
dayjs.utc().format('HH:mm:ss z')
'14:24:45 GMT+1'
```

how can I make dayjs not display this `z` timezone as `GMT+1`, but instead show `UTC`?

I found this call:
```
dayjs.utc().tz('UTC').format('HH:mm:ss z')
'14:26:03 UTC'
```
but I'd like to find a way to make dayjs in general to work with the `UTC` timezone, without have to convert every individual timestamp.

**Expected behavior**

I would like instructions and/or updated documentation to show how to make dayjs show the `z` timezone as `UTC` for a `utc()` timestamp.

**Information**
- Day.js Version: `1.10.8`
- OS: `Linux`
- Browser: `Chrome 99` & `nodejs 16.14.0`
- Time zone: `UTC`

Start with the utc, timezone, and advancedFormat plugin usage shown in the issue, then check the related Day.js formatting and timezone documentation. Confirm how `z` behaves for `dayjs.utc()` and document the supported way to display `UTC`, including whether converting with `.tz('UTC')` is required; done means the documented behavior answers the reported example.

Written by the indexing model from the issue text.