Update GitHub API authentication in release scripts

Per the example in https://github.com/octokit/core.js/#rest-api-example
the value for the `auth` property in the `Octokit` constructor is now just
the personal access token, not `token $TOKEN`. The old format still
works with a recently generated token though.

To test:

```sh
export GITHUB_TOKEN=$TOKEN
node scripts/generate-change-list.js $PREV_CLIENT_VERSION
```

Where `$TOKEN` is a token generated at https://github.com/settings/tokens and
`$PREV_CLIENT_VERSION` is the previous released version of the client to
use as a starting point for the change list.

scripts/create-github-release.js

@@ -29,7 +29,7 @@ async function createGitHubRelease() {
   }
 
   const octokit = new Octokit({
-    auth: `token ${process.env.GITHUB_TOKEN}`,
+    auth: process.env.GITHUB_TOKEN,
   });
 
   const changes = await changelistSinceTag(octokit);

scripts/generate-change-list.js

@@ -137,7 +137,7 @@ async function changelistSinceTag(octokit, tag = getHighestVersionTag()) {
 
 if (require.main === module) {
   const { Octokit } = require('@octokit/rest');
-  const octokit = new Octokit({ auth: `token ${process.env.GITHUB_TOKEN}` });
+  const octokit = new Octokit({ auth: process.env.GITHUB_TOKEN });
   const tag = process.argv[2] || getHighestVersionTag();
 
   changelistSinceTag(octokit, tag)