Update "README.md" and osm2pgsql.org for latest best practice (e.g. jsonb versus hstore)?

[mboeringa]

Minor issue, but I noticed the repository's "README.md" still only mentions hstore for tag storage, and the osm2pgsql.org site still suggests the need to install the hstore extension (https://osm2pgsql.org/doc/manual.html#preparing-the-database) in the main section about preparing the database, even though the options for json and jsonb are clearly mentioned multiple times in other sections.

Although hstore is still a perfectly fine and working option for osm2pgsql, the need to install the extra hstore extension versus native json/jsonb support in PostgreSQL makes it a bit more complicated than necessary, and the new flex config files seem to be using the jsonb option by default (#1496).

Maybe it's time to do a small review of both this repository and the site to have it reflect the latest best practices, or at least present the equivalent and new json options everywhere hstore is mentioned?

Relevant but now outdated previous discussions:
#672
#738
#1496