chore(ci): Always run backend system tests on main pushes (JAVA-519)

Keep path filters on pull_request only so Android-only PRs skip the job,
but every merge to main still runs backend system tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: 0xadam-brown

.github/workflows/system-tests-backend.yml

@@ -4,10 +4,6 @@ on:
   push:
     branches:
       - main
-    paths-ignore:
-      - '*android*/**'
-      - 'sentry-compose/**'
-      - 'sentry-samples/sentry-samples-android/**'
   pull_request:
     paths-ignore:
       - '*android*/**'