Fix dependabot auto-merge (#245)

`fastify/github-action-merge-dependabot` filters out PRs triggered not
by `pull_request` event. After creating a ticket to the action repo,
developers answered me that probably there is no need to use
`pull_request_target`:
fastify/github-action-merge-dependabot#474 (review).

And it seems we don't: the workflow is targeted on dependabot PRs only,
which are internal (not from forked repos), so for the `pull_request`
event the workflow will have all required write permissions.

.github/workflows/dependabot-auto-merge.yml

@@ -1,5 +1,5 @@
 name: Dependabot
-on: pull_request_target
+on: pull_request
 
 permissions:
   pull-requests: write