+00:00 timezone bug

**Describe the bug**

When we have +00:00 timezone offset like 'Europe/London" dayjs gets users timezone offset, which causes incorrect time showing in different cases.

Example

Current behavior:
dayjs('2021-10-31T15:00:00+00:00').tz('Europe/London').format() // 2021-10-31T15:00:00Z

**Expected behavior**
Behavior after fix:
dayjs('2021-10-31T15:00:00+00:00').tz('Europe/London').format() // 2021-10-31T15:00:00+00:00

**Information**
- Day.js Version [v1.11.20]
- OS: [iOS]
- Browser [Chrome 147]
- Time zone: [BST]

Start by running the reported dayjs timezone example with the +00:00 offset and Europe/London, then trace the timezone parsing and formatting entry points involved. The fix is complete when the example preserves +00:00 instead of formatting as Z, with a regression test covering this case.

Written by the indexing model from the issue text.