Flex version of the osm2pgsql configuration

[joto]

This commit contains one file with an osm2pgsql configuration for the flex output that can be used instead of the old configuration for the pgsql output. It replaces the openstreetmap-carto.style and openstreetmap-carto.lua files.

The configuration is nearly 100% compatible to the old one.

The database layout will be exactly the same with just very little changes. The id columns (osm_id) and geometry columns (way) on all tables will get the NOT NULL flag when using the flex output. These have always been NOT NULL in practice anyway.

The content of the database will be the same with only minor irrelevant differences.

Run like this: osm2pgsql -O flex --style openstreetmap-carto-flex.lua -d gis ~/path/to/data.osm.pbf

See #4977

[joto]

I have

• rebased the PR
• removed the index code from the flex config file (see discussion in #4977)
• added an extra commit which does the actual change, i.e. removes the old config, updates docs and scripts and Github CI

I have not touched the stuff in scripts/lua. It mentions old-style multipolygons and the last change is from 2020, so I am not sure how relevant that still is. It will not work with the new config file and making it work would be a larger effort.