task: audit yarn resolutions – flat

[toufali]

Because

• Yarn resolutions should be used as last resort
• pinning old packages prohibits security patches

This pull request

• removes the resolution for flat package

Issue that this pull request solves

Closes: FXA-11867

Other information

I added this resolution after running into issues a couple months ago while bulk-updating packages. We've done numerous upgrades and resolution removals since then, and I'm hoping it "just works". After removing the resolution, the package still resolves at 5.0.2 (no change), suggesting this removal is safe.

[dschom]

Seems good to me. test-unit just needs a mkdir -p var/public in the command. I'm geussing what's happening is that the prebuild creates the folder, but since the folder is empty it doesn't get added to the cache. Maybe circle-ci's workspace, or maybe this is a side effect of the new nx cache changes ... /nosture) At any rate, I think wants that's cleared up, this seems great, so pre-emptive r+.

[toufali]

Thanks @dschom – I fixed the broken tests via #19028 and rebased this PR.