Update documentation for the PostgreSQL 9.6 and 10 removals

README.md

doc/administration/database_load_balancing.md

doc/administration/geo/replication/database.md

doc/administration/geo/replication/external_database.md

doc/administration/geo/replication/index.md

doc/administration/geo/replication/security_review.md

doc/administration/gitaly/praefect.md

doc/administration/high_availability/database.md

doc/administration/high_availability/pgbouncer.md

doc/ci/docker/using_docker_images.md

doc/development/filtering_by_label.md

doc/development/geo.md

doc/development/namespaces_storage_statistics.md

doc/development/verifying_database_capabilities.md

doc/install/requirements.md

doc/topics/autodevops/customize.md

doc/topics/autodevops/stages.md

doc/topics/autodevops/upgrading_postgresql.md

doc/user/clusters/crossplane.md