fix: Format YY should never return one-digit years

[flaviendelangle]

I noticed this issue while working on https://github.com/mui/mui-x:

console.log(dayjs().set('year', 2009).format('YY')); // 09
console.log(dayjs().set('year', 9).format('YY')); // 9

If padStart is not widely supported enough I can of course implement the same logic manually.