Unvendor Yarn; favor Corepack for installation

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.

.yarnrc.yml

@@ -15,5 +15,3 @@ nodeLinker: node-modules
 plugins:
   - path: .yarn/plugins/@yarnpkg/plugin-allow-scripts.cjs
     spec: "https://raw.githubusercontent.com/LavaMoat/LavaMoat/main/packages/yarn-plugin-allow-scripts/bundles/@yarnpkg/plugin-allow-scripts.js"
-
-yarnPath: .yarn/releases/yarn-4.1.1.cjs

README.md

@@ -49,7 +49,7 @@ See our documentation:
 
 - Install the current LTS version of [Node.js](https://nodejs.org)
   - If you are using [nvm](https://github.com/creationix/nvm#installation) (recommended) running `nvm install` will install the latest version and running `nvm use` will automatically choose the right node version for you.
-- Install [Yarn v3](https://yarnpkg.com/getting-started/install)
+- Install [Yarn](https://yarnpkg.com) v4 via [Corepack](https://github.com/nodejs/corepack?tab=readme-ov-file#how-to-install)
 - Run `yarn install` to install dependencies and run any required post-install scripts
 
 ### Testing and Linting