Upgrade to pnpm@8; drop support for Node.js 14 (OSOE-603)

[0liver]

This should be done once we've upgraded all internal projects and developers to Node.js 16, at least.

There's at least one catch, though. Citing the v8.0.0 release notes:

A lockfile is generated even for projects with no dependencies.

Also, we should wait a bit until at least a few bug fixes have been incorporated.

Jira issue

[Piedone]

Together with this, the NE reference in Lombiq.Tests.UI also needs to be removed, because that's also only there for PNPM 7 compatibility during NuGet publishing.