Migrate uiSettings defintions to NP

[mshustov]

• Migrate platform-specific uiSettings definitions to the NP. Other definitions should move to corresponing plugins
  https://github.com/elastic/kibana/blob/master/src/legacy/core_plugins/kibana/ui_setting_defaults.js
• Add validation for all platform-owned uiSettings

List from #63459:

• defaultRoute 👉 core
• dateFormat 👉core (date format is something super central, IMHO we shouldn't split these up but put everything into core)
• dateFormat:tz 👉core
• dateFormat:scaled 👉core
• dateFormat:dow 👉core
• dateNanosFormat 👉core
• truncate:maxHeight 👉core
• theme:darkMode 👉core
• notifications:banner 👉core
• notifications:lifetime:banner 👉core
• notifications:lifetime:error 👉core
• notifications:lifetime:warning 👉core
• notifications:lifetime:info 👉core
• accessibility:disableAnimations 👉core

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)