Inconsistent behaviour of diff(.., 'months) on leap days

Bug description

The difference in months between two days yields different results depending on the time of the day on leap days.

```
dayjs = require('dayjs')
customParseFormat = require('dayjs/plugin/customParseFormat')
dayjs.extend(customParseFormat)

dayjs('2024-05-29T21:00:00.000Z').diff(dayjs('2024-02-29T00:00:00.000Z'), 'months') // Result = 3
dayjs('2024-05-29T22:00:00.000Z').diff(dayjs('2024-02-29T00:00:00.000Z'), 'months') // Result = 2 <-- this one seems wrong
dayjs('2024-05-29T23:00:00.000Z').diff(dayjs('2024-02-29T00:00:00.000Z'), 'months') // Result = 3
```

Expected behaviour:

The same test on a non leap day:

```
dayjs('2024-05-28T21:00:00.000Z').diff(dayjs('2024-02-28T00:00:00.000Z'), 'months') // Result = 3
dayjs('2024-05-28T22:00:00.000Z').diff(dayjs('2024-02-28T00:00:00.000Z'), 'months') // Result = 3
dayjs('2024-05-28T23:00:00.000Z').diff(dayjs('2024-02-28T00:00:00.000Z'), 'months') // Result = 3
```

**Information**
- Day.js Version 1.11.10
- OS: Windows 10 Enterprise (same result within Ubuntu docker image)
- Browser Nodejs Repl v21.1.0 and Chrome 121.0.6167.18
- Time zone: GMT+01:00

Start by running the reported Node.js reproduction with Day.js 1.11.10, then locate the implementation of diff(..., 'months') and its existing tests. Compare the leap-day and non-leap-day cases across the three times shown. Done means the leap-day examples behave consistently with the expected three-month result and the regression is covered by a test.

Written by the indexing model from the issue text.