Commit 7117685461af50f50c03f43e6a622284c8d54694 made pg_start_backup parallel-restricted rather than parallel-safe, because it now relies on backend-private state that won't be synchronized with the parallel worker. However, it didn't update pg_proc.h. Separately, Andreas Karlsson observed that system_views.sql neglected to reiterate the parallel-safety markings whe redefining various functions, including this one; so add a PARALLEL RESTRICTED declaration there to match the new value in pg_proc.h.