Change APIs to classicpress.net endpoints

[viktorix]

• [ ] /checksums/
• [ ] /core/importers/1.0/
• [ ] /core/stable-check/1.0/
• [ ] /core/version-check/1.0/
• [ ] /events/1.0/
• [ ] /features/1.0/
• [ ] /secret-key/1.0/salt/
• [ ] /migration/
• [ ] /translations/core/1.0.0/translations.json
• [ ] /twemoji/
• [ ] /upgrade/

Note: After a quick review, WordPress changed certain API functions. Some of them need more than a URL swap.