Remove yarn dependency from example-contracts AND CI

[richardpringle]

Context and scope
While yarn might be better than NPM, I really don't think it makes sense to have an extra dependency. Those who prefer yarn will know how to do yarn equivalents when it comes to documentation.

for CI, we should use the command npm ci which will grab all the exact dependencies that are in the package-lock.json.

Discussion and alternatives
Open to alternatives, but the fewer external dependencies, the better IMO.

Open questions
N/A

[richardpringle]

@ceyonur, I assigned this to you... but that doesn't mean you need to do it haha. If you don't have time or whatever, feel free to assign it back to me and I'll get to it at some point in the future.

[ceyonur]

will be closed by #672

[richardpringle]

🎉