avoid deprecation warnings about listForRepo

```
DEPRECATED (@octokit/rest): `issues.getForRepo()` is deprecated, use `issues.listForRepo()`
```

Signed-off-by: Jakub Sokołowski <jakub@status.im>

bot_scripts/greet-new-contributor.js

@@ -54,7 +54,7 @@ async function greetNewContributor (context, robot) {
   robot.log(`${botName} - Handling Pull Request #${prInfo.number} on repo ${repoInfo.owner}/${repoInfo.repo}`)
 
   try {
-    const ghissuesPayload = await github.issues.getForRepo({
+    const ghissuesPayload = await github.issues.listForRepo({
       ...repoInfo,
       state: 'all',
       creator: payload.pull_request.user.login