Fix lsp-org compatibility with Org 9.7

[aavanian]

With org-mode >= 9.7, the api to access some node data has changed and requires to use a new function org-element-property. This fixes #4300.

The fix is a simple one-liner but, in order to preserve the functionality for org-mode < 9.7:

• we define a variable to indicate which api we need to use
• we initialize that variable when org-element is loaded by testing it
• we branch between the two ways of accessing the needed property depending on the variable when lsp-org is run

Thanks to @fanshi1028 for the more elegant one-liner.