fix: build PostGIS from the stable-3.6 branch

[NiccoloFei]

Build PostGIS using stable-3.6 instead of main.
The main branch has moved the address_standardizer extension into a separate https://github.com/postgis/address_standardizer repository.

When building address_standardizer from its new repository, the library is being created as address_standardizer.so , while in v18 packages the extension was called address_standardizer-3.so.

This makes pg_upgrade complain with:

could not load library "address_standardizer-3": ERROR: could not access file "address_standardizer-3": No such file or directory 
In database: app

As a workaround, pin PostGIS to stable-3.6 until we have a solution.

[NiccoloFei]

A green E2E run here: https://github.com/cloudnative-pg/postgres-trunk-containers/actions/runs/23438542226