[gdb] Update ThreadExitedEvent parent class for 17.0 (#14729)

Even though the parent thread for `ThreadExitedEvent` won't change from
`Event` to `ThreadEvent` until 17.0 is released, this actually makes
type checking work better in practice for all released versions that
support `ThreadExitedEvent` because at run time `ThreadExitedEvent` has
the lone attribute, `inferior_thread`, that it would have inherited from
`ThreadEvent`.

Upstream GDB issue:
* https://sourceware.org/bugzilla/show_bug.cgi?id=33444

Author: malsyned

stubs/gdb/gdb/__init__.pyi

@@ -960,7 +960,7 @@ class ExitedEvent(Event):
     exit_code: int
     inferior: Inferior
 
-class ThreadExitedEvent(Event): ...
+class ThreadExitedEvent(ThreadEvent): ...
 
 class StopEvent(ThreadEvent):
     details: dict[str, object]