Timezone bug: wrong behavior with strings containing timezone infos compared to momentjs

**Describe the bug**
The timezone plugin behaves wrong compared to the respective `moment` plugin.

Example: I am working with an external API, which returns a date in a string format like this: `Wed, 16 Aug 2023 10:47:24 +0200`

As you can see, this string already contains a timezone information, in this case UTC+2.

We have a setup internally, where we always try to first parse any date in respect to the users timezone - in this case, the user, which is "connected" to this API call, is in the `Europe/Vienna` timezone --> UTC+2 is already the correct timezone.

Consider this code:

```js
moment.tz('Wed, 16 Aug 2023 10:47:24 +0200', 'Europe/Vienna').utc().format()
// returns: '2023-08-16T08:47:24Z'

// intentionally trying to pass a completely wrong timezone:
moment.tz('Wed, 16 Aug 2023 10:47:24 +0200', 'Asia/Taipei').utc().format()
// ALSO returns: '2023-08-16T08:47:24Z'

// WHY? because the "input string" already contains timezone info --> the 2nd param is kind of ignored and both lines of code return the same value.
// in my opinion this is the correct behavior, because why would moment consider the 2nd param, if the 1st param already contains proper timezone infos?

// dayjs:
dayjs.tz('Wed, 16 Aug 2023 10:47:24 +0200', 'Europe/Vienna').utc().format()
// returns: '2023-08-16T06:47:24Z'
// in my opinion this is wrong, because the "timezone conversion" actually happens twice, which does not make any sense to me.
```

This current behavior is really weird, because the moment vs. dayjs behavior is identical **if** the first param for `.tz()` is either a proper JS Date or a String WITHOUT timezone infos. But in this special case (a string WITH timezone infos), the behavior suddenly is really different. This resulted in a lot of wrong dates in our codebase since we switched from moment to dayjs.

**Expected behavior**
Calling `dayjs.tz()` with a string in a format already containing timezone infos, the 2nd param of `.tz()` should actually be ignored.

**Information**
- Day.js Version v1.11.9
- OS: nodejs
- Browser -
- Time zone: Vienna Austria

Start at the timezone plugin entry point behind dayjs.tz and reproduce the supplied string-with-offset example in Node.js. Compare its output with moment.tz and the existing behavior for Date inputs and strings without timezone information. Done means timezone-bearing strings produce the expected UTC result without applying the supplied timezone twice.

Written by the indexing model from the issue text.