fix: forwards support for fs.rm in node 14.14+

[lukebro]

In node v14.14+ a deprecation warning is now always given:

[DEP0147] DeprecationWarning: In future versions of Node.js, fs.rmdir(path, { recursive: true }) will be removed. Use fs.rm(path, { recursive: true }) instead
(Use node --trace-deprecation ... to show where the warning was created)
nodejs.org/api/deprecations.html#DEP0147

So I've checked for the existence of fs.rm in favor of fs.rmdir if available, which should support v16+ but also v12.

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[lukebro]

@googlebot I signed it!

[paulshryock]

Let's do this.

[brendankenny]

Sorry for the delay, LGTM. Would love to have this in for the next Lighthouse release.