chore(deps): update postgres docker tag to v15

.drone.yml

@@ -63,7 +63,7 @@ services:
       - name: tmp-mysql-migration
         path: /var/lib/mysql
   - name: test-postgres-unit
-    image: postgres:14
+    image: postgres:15
     environment:
       POSTGRES_PASSWORD: vikunjatest
       POSTGRES_DB: vikunjatest
@@ -73,7 +73,7 @@ services:
     commands:
       - docker-entrypoint.sh -c fsync=off -c full_page_writes=off # turns of wal
   - name: test-postgres-integration
-    image: postgres:14
+    image: postgres:15
     environment:
       POSTGRES_PASSWORD: vikunjatest
       POSTGRES_DB: vikunjatest
@@ -83,7 +83,7 @@ services:
     commands:
       - docker-entrypoint.sh -c fsync=off -c full_page_writes=off # turns of wal
   - name: test-postgres-migration
-    image: postgres:14
+    image: postgres:15
     environment:
       POSTGRES_PASSWORD: vikunjatest
       POSTGRES_DB: vikunjatest