Migrate to modern Yarn version

Starting with version 2 Yarn uses a new lockfile format and changes how modules are resolved by Node.js when the PnP linker is used.

If the PnP linker should cause any compatibility issues in the future, we can simply switch back to using a node_modules folder by setting nodeLinker: "node-modules" in .yarnrc.yml.

Furthermore, the package name has been changed to arsnova-webclient to be consistent with the repository/project name. The version property has been removed since it was not maintained and is not useful for a private package which is not published.

This also disables dependency caching for CI.

Closes: #1058 (closed)
See: https://yarnpkg.com/features/pnp
Related: particify/dev/common-concerns#47