postgres: in the datasource-plugin, switch from lib/pq to pgx

[gabor]

the current version of lib/pq has problems ( see #65816, caused by a bug in lib/pq at lib/pq#1106 ), and the project itself is in maintenance mode, meaning there's little chance that this gets fixed. the project itself recommends migration to https://github.com/jackc/pgx , so this is what we will do.

we did some initial tests, and the pgx works fine, with some minor issues, that we can fix in our code.
(NOTE: the lib/pq module is also used by grafana-backend, but this issue only deals with switching to pgx in the postgres datasource plugin.)

[gabor]

we had to revert the fix ( #83760 ) , we found certain complications, will have to adjust the code and merge it again.