bug in dayjs month

**Describe the bug**
A clear and concise description of what the bug is.

**Expected behavior**
A clear and concise description of what you expected to happen.

**Information**
- Day.js Version v1.11.7
- OS: Windows 11
- Time zone: GMT+02:00


```js
console.log(dayjs());
```
is returning `M {
'$L': 'en',
'$d': 2023-06-06T13:06:06.731Z,
'$x': {},
'$y': 2023,
'$M': 5,
'$D': 6,
'$W': 2,
'$H': 15,
'$m': 6,
'$s': 6,
'$ms': 731
}`
Shouldn't $M be 6

The payload names no source file or test. Start by checking Day.js's public month API and the meaning of internal `$M`; reproduce the snippet under the stated version, then compare the observed value with documented behavior and add a focused regression test only if it is a real defect.

Written by the indexing model from the issue text.