Preparation for Shlink 4.0.0

[acelaya]

• [ ] Dynamic rule-based redirects
• [x] Remove deprecated stuff
  • [x] General stuff -> https://github.com/shlinkio/shlink/pull/1999
  • [x] URL Validation -> https://github.com/shlinkio/shlink/pull/2012
  • [x] Remove support for openswoole
• [x] Document migration from v3 to v4 in UPGRADE.md
• [x] Make PHP 8.3 the default
• [x] Make API v2 behave like API v3