Feat: remove yarn2 support

[orsagie]

• [ ] Tests written and linted
• [ ] Documentation written / README.md updated https://snyk.io/docs/snyk-for-node/
• [ ] Follows CONTRIBUTING agreement
• [ ] Commit history is tidy https://git-scm.com/book/en/v2/Git-Branching-Rebasing
• [ ] Reviewed by Snyk team

What this does

Removes yarn2 support as it is breaking snyk for yarn users on node8.

Yarn2 requires node>10. We still have users on node8 using yarn and ever since we added yarn2 they are no longer able to add snyk to their projects.

Since snyk still supports node8, we must unfortunately remove our support for yarn2.

Node8 support will be dropped in the near future, after which we can reinstate yarn2 support. In the meantime, users can pin the version 1.385.0 to retain yarn 2 support

See relevant issue: https://github.com/snyk/snyk/issues/1270

[aviadhahami]

@orsagie linted and force-pushed

[snyksec]

:tada: This PR is included in version 1.28.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: