[Bug Report] element-ui is not compatible with node-sass v6

### Element UI version
2.15.6

### OS/Browsers version
any

### Vue version
2.6.14

### Reproduction Link
https://github.com/ElemeFE/element/blob/dev/package.json#L124

### Steps to reproduce
- Upgrade node-sass to v6 (in order to support Node.js 16)
- yarn/npm install

### What is Expected?
It should install the package without problems when running Node.js 16

### What is actually happening?
It breaks due to the version being stuck at node-sass v4