[BUG] dayjs "YYMMDD" Parsing, Formatting Issue

**Describe the bug**
```typescript
const startDate = '230701'
const endDate = '230702';

dayjs(startDate, 'YYMMDD').startOf('day').format() // '2307-01-01T00:00:00+09:00'
dayjs(endDate, 'YYMMDD').endOf('day').format() // '2307-02-01T23:59:59+09:00'

dayjs(startDate, 'YYMMDD').startOf('day').format('YYMMDD') // '070101'
dayjs(endDate, 'YYMMDD').endOf('day').format('YYMMDD') // '070201'
```

How Can I solve this problem?

**Expected behavior**
I think it should be
`20230701T00:00:00+09:00`
`20230702T23:59:59+09:00`
`230701`
`230702`
in order from top to bottom

**Information**
- Day.js Version "^1.11.7"
- OS: MacOS
- Browser: X (in express server)
- Time zone: UTC+09:00 KST (Korean Standard Time)

Start by reproducing the YYMMDD parsing and formatting examples from the issue in Day.js. Trace the date parsing and format-token handling used for YY and MMDD, then verify that startOf/endOf produce the expected 2023 dates and that formatting returns 230701 and 230702.

Written by the indexing model from the issue text.