Correct the documentation of extension building infrastructure

Hi hackers,

I noticed that the documentation of extension building infrastructure [1]
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] 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]: https://www.postgresql.org/message-id/flat/CAMsr%2BYFsCMH3B4uOPFE%2B2qWM6k%3Do%3Dhf9LGiPNCfwqKdUPz_BsQ%40mail.gmail.com


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