org-persist uses Emacs 28+ buffer-local-boundp

[Kyle Meyer]

Hi Ihor,

27215ac5b (org-persist: Handle undefined variables in elisp containers,
2022-02-13) added a call to buffer-local-boundp, which was introduced in
(what will be) Emacs 28.1.

Could you update org-persist-write:elisp to be compatible with older
Emacs versions (e.g., by inlining the short definition and adding a
comment about why buffer-local-boundp isn't used)?

Thanks.