Drop parse `yyyy-Wnn` format?

**Describe the bug**
Parsing Format `yyyy-Wnn` seems to be missing in the last version

**Expected behavior**

```js
const dayjs = require('dayjs')
const utc = require('dayjs/plugin/utc');
const weekOfYear = require('dayjs/plugin/weekOfYear')
dayjs.extend(weekOfYear);
dayjs.extend(utc);
dayjs.utc('2020-W03').valueOf();
```

`NaN` with `“dayjs”: “^1.11.0"`
but `1577847600000` with `“dayjs”: “^1.10.4"`

I don't see anything relavant in the changelog (but I might be wrong).
Do I miss something? Do I have to add a plugin?

Thanks in advance

**Information**
- Mac OS/X
- Node.js v16.1.0
- Time zone: GMT

Start by running the provided Node.js reproduction with Day.js 1.11.0 and compare its result with 1.10.4. Trace the date-parsing entry point and relevant week or UTC plugins, then verify that parsing `2020-W03` produces the expected timestamp or that the supported usage is clearly documented.

Written by the indexing model from the issue text.