Correct the documentation of extension building infrastructure

Hi hackers,

I noticed that the documentation of extension building infrastructure [1]https://www.postgresql.org/docs/12/extend-pgxs.html
has a paragraph about isolation tests since v12, but they actually can be
run since v14 only.

Actual isolation tests support for extensions built with PGXS was added
in [2]/messages/by-id/CAMsr+YFsCMH3B4uOPFE+2qWM6k=o=hf9LGiPNCfwqKdUPz_BsQ@mail.gmail.com and it was agreed there that it should not be backpatched. How
about fixing the documentation for v12 and v13 then?

[1]: https://www.postgresql.org/docs/12/extend-pgxs.html
[2]: /messages/by-id/CAMsr+YFsCMH3B4uOPFE+2qWM6k=o=hf9LGiPNCfwqKdUPz_BsQ@mail.gmail.com
/messages/by-id/CAMsr+YFsCMH3B4uOPFE+2qWM6k=o=hf9LGiPNCfwqKdUPz_BsQ@mail.gmail.com

Best regards,
Karina Litskevich
Postgres Professional: http://postgrespro.com/