According to the Yarn documentation, Yarn no longer needs to be bundled
with a project, but rather, installing Yarn ought to be done using
[Corepack](https://yarnpkg.com/corepack), which is shipped in modern
Node versions. This commit removes the Yarn binary from the repo and
updates the installation instructions in the README to match.

* ci: run `corepack enable` as part of prepare

* chore(ci): do redundant setup-node step before checkout to enable yarn v4 before deps install

- makes nodejs version `lts/*` explicit in ci workflow