.github: correct the regex in mileston workflow

also use pull_request_target event so the action is run in the
context of the base of the pull request. this helps us to overcome
the "Resource not accessible by integration" issue where the action
is run in the context of the pull request.

Signed-off-by: Kefu Chai <kchai@redhat.com>

.github/milestone.yml

@@ -1,6 +1,6 @@
 base-branch:
-  - "luminous"
-  - "nautilus"
-  - "octopus"
-  - "pacific"
-  - "quincy"
+  - "(luminous)"
+  - "(nautilus)"
+  - "(octopus)"
+  - "(pacific)"
+  - "(quincy)"

.github/workflows/milestone.yml

@@ -1,9 +1,6 @@
 ---
-name: "Pull Request Add Milestone"
-on:
-  pull_request:
-    types:
-      - opened
+name: "Pull Request Set Milestone"
+on: pull_request_target
 jobs:
   milestone:
     runs-on: ubuntu-latest