Update Node.js to v14.x (#432)

Node.js v14.x or later is required by markdownlint-cli v0.33.0, which
is the current version which is installed with `npm install purescript
spago markdownlint-cli`.

https://github.com/igorshubovych/markdownlint-cli/blob/b9158d3e9c36496bfaef9ce98f886d9117c6f502/package.json#L10

Author: gemmaro

.github/workflows/tests.yml

@@ -26,7 +26,7 @@ jobs:
       - name: Set up Node toolchain
         uses: actions/setup-node@v1
         with:
-          node-version: "12.x"
+          node-version: "14.x"
 
       - name: Cache NPM dependencies
         uses: actions/cache@v2