v4.40.0-rc10: Refactor Custom App Settings

What's Changed

• refactor(settings)!: fold custom_app_settings → settings_custom by @wesleyboar in #1177
• docs: v4.40.0-rc10 by @wesleyboar in 9e80bc56,
  2c096724, 383d5c73
• fix(settings): restore custom_app_settings for v4.40 rollout by @wesleyboar in #1198

Full Changelog: v4.40.0-rc9...v4.40.0-rc10