main/postgresql15: new aport

https://www.postgresql.org/about/news/postgresql-15-released-2526/

TODO:

• All pipelines green.
• Change _default_ver in main/postgresql14 to false and rebuild.
• Rebuild all packages that depend on postgresql-dev.
• Update comment about ICU locales in postgresql.confd in main/postgresql-common.
• Consider generating glibc-like aliases for ICU locales (en_US -> en-US-x-icu).