Add Postgres version requirement

[lognaturel]

Making this implicit requirement explicit!

[lognaturel]

Making a couple of quick cosmetic changes while here, hopefully that's ok! For better or worse, I think Central stopped being minimal a little while ago!

We used to use the "It is currently under development" language when we had some dead repos but now those have been removed.

[matthew-white]

Just to document one reason why Postgres 14+ is required, @lognaturel wrote on Slack:

From @eyelidlessness about postgres 14 now being required: it’s about keywords that now can be used as column labels without AS

-- Syntax error in 12, 13
SELECT '' action;

-- No syntax error in 14+
SELECT '' as action;
SELECT '' "action";