closes https://github.com/TryGhost/Ghost/issues/12026
- 3 new portal settings were added in `3.23` in default settings - `portal_button_style`, `portal_button_icon`, `portal_button_signup_text`
- New settings default to group `core` on migrating from pre 3.22 versions due to missing columns in DB
- Migration here updates the new settings to correct group
