fix(validate-pr-title): use PRT not PR (#18)

## Which problem is this PR solving?

- Private repos cannot be monitored by the semantic convention enforcer

## Short description of the changes

See
amannn/action-semantic-pull-request#173 (comment)

however, this by itself isn't enough as per
honeycombio/loadgen#27 - permissions may also
need to explicitly be granted.

.github/workflows/validate-pr-title.yml

@@ -1,7 +1,7 @@
 name: "Validate PR Title"
 
 on:
-  pull_request:
+  pull_request_target:
     types:
       - opened
       - edited