Update README.md to update default Node version to 20 (#949)

The README's default example for setting up Node use version 16, which is at end of life. This PR updates those examples to use 20.

Author: bengreeley

README.md

@@ -103,7 +103,7 @@ The `node-version` input supports the Semantic Versioning Specification, for mor
 
 Examples:
 
- - Major versions: `16`, `18`, `20`
+ - Major versions: `18`, `20`
  - More specific versions: `10.15`, `16.15.1` , `18.4.0`
  - NVM LTS syntax: `lts/erbium`, `lts/fermium`, `lts/*`, `lts/-n`
  - Latest release: `*` or `latest`/`current`/`node`
@@ -135,7 +135,7 @@ steps:
 - uses: actions/checkout@v4
 - uses: actions/setup-node@v4
   with:
-    node-version: 16
+    node-version: 20
     cache: 'npm'
 - run: npm ci
 - run: npm test
@@ -148,7 +148,7 @@ steps:
 - uses: actions/checkout@v4
 - uses: actions/setup-node@v4
   with:
-    node-version: 16
+    node-version: 20
     cache: 'npm'
     cache-dependency-path: subdir/package-lock.json
 - run: npm ci
@@ -185,7 +185,7 @@ To get a higher rate limit, you can [generate a personal access token on github.
 uses: actions/setup-node@v4
 with:
   token: ${{ secrets.GH_DOTCOM_TOKEN }}
-  node-version: 16
+  node-version: 20
 ```
 
 If the runner is not able to access github.com, any Nodejs versions requested during a workflow run must come from the runner's tool cache. See "[Setting up the tool cache on self-hosted runners without internet access](https://docs.github.com/en/[email protected]/admin/github-actions/managing-access-to-actions-from-githubcom/setting-up-the-tool-cache-on-self-hosted-runners-without-internet-access)" for more information.