Emacs 31 obsolete function warnings

I am not sure whether this can be handled now. But when it is released this can be a problem.


```
exec-path-from-shell.el:151:22: Warning: ‘cl-gensym’ is an obsolete function
    (as of 31.1); use ‘gensym’ instead.
exec-path-from-shell.el:152:27: Warning: ‘cl-gensym’ is an obsolete function
    (as of 31.1); use ‘gensym’ instead.
```