Add required minimum node version to readme

[line-o]

it should clearly state the minimum required version

which is >= 18.19

[DrRataplan]

Already there, listing Node 18 as the minimal: https://github.com/eXist-db/xst/blob/main/readme.md?plain=1#L22

[line-o]

I have a hard time coming up with a catchy and human readable equivalent of

>= 18.19 || >=20.5

[line-o]

The minimum version is 20 by now