This addresses https://security.snyk.io/vuln/SNYK-GOLANG-GOPKGINYAMLV3-2841557
by updating yaml v3 to latest version.
It also stops using yaml v2 directly (we were using both v2 and v3 mixed).
We still rely on yaml v2 indirectly, via urfave cli, though.
Note that the security vulnerability does not affect v2.