update to postgres 15rc1

[ImreSamu]

"PostgreSQL 15 RC 1 Released!" https://www.postgresql.org/about/news/postgresql-15-rc-1-released-2516/

STEPS:

• git mv 15beta4-3.3 15rc1-3.3
• git mv 15beta4-master 15rc1-master
• make update
• modify
  • README.md
  • .github/workflows/main.yml

[ImreSamu]

Ready to review / merge