[CI / OAS] Use 'pull_request_target' for Bump action

In order for the Bump.sh action to be able to post github comments on
PRs opened from forks, the GH action needs to listen to the
'pull_request_target' event (instead of the 'pull_request' event).

More details about the introduction of this event in this GH blog
post:
https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/

Complete documentation about this event:
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target

.github/workflows/bump.yml

@@ -7,7 +7,7 @@ on:
     paths:
       - 'oas_docs/kibana.serverless.yaml'
 
-  pull_request:
+  pull_request_target:
     branches:
       - main
     paths:
@@ -33,7 +33,7 @@ jobs:
           file: oas_docs/kibana.serverless.yaml
 
   api-diff:
-    if: ${{ github.event_name == 'pull_request' }}
+    if: ${{ github.event_name == 'pull_request_target' }}
     name: Check API diff on Bump.sh
     runs-on: ubuntu-latest
     steps: