upgrade to PostGIS 3.4

[ImreSamu]

Upgrade to PostGIS 3.4

Changes from upstream:

• https://postgis.net/2023/08/PostGIS-3.4.0/
  • PostGIS version 3.4.0 has been released. This version is compatible with PostgreSQL versions 12-16.
• pgsql-pkg-debian: postgis updated to version 3.4.0+dfsg-1.pgdg+1 -
  • The package postgis was updated on apt.postgresql.org.

The following updates have been made:

• For PostgreSQL versions 12-16, PostGIS has been upgraded from version 3.3 to 3.4, using the git mv command.
• The Dockerfiles have been updated using the ./update.sh script.
• The README.md file has been updated manually.
• In the Makefile, the LATEST_VERSION has been manually updated to 15-3.4.
  • VERSION=15-3.4 VARIANT=default make -n push looks OK
• The .github/workflows/main.yml file has been updated manually.

PostgreSQL 11 is not compatible with PostGIS 3.4, so it remains on PostGIS 3.3.

[ImreSamu]

Ready for review and merging.

[ImreSamu]

README improved. and "All checks have passed".

Ready for review and merging.