Add validation constraints to all system.* simple config (except system.rss)

Problem/Motivation

Per #2952037: [meta] Add constraints to all simple configuration, the current state of validatable simple config in the System module is:

• system.site
• ✅system.maintenance
• system.cron
• system.date
• system.diff
• system.logging
• system.performance
• system.rss
• system.theme
• system.file
• ✅system.image
• ✅system.image.gd
• system.mail
• system.advisories
• ✅system.feature_flags

All of these seem possible to make fully validatable. Sole exception: system.rss, because it A) depends on config entities, B) it depends on multiple or none at the same time. It probably makes more sense to remove that config 😅

P.S.: system.theme.global isn't in that list because it extends type: theme_settings, not type.config, but fortunately #3416178: Add validation constraints to `type: theme_settings` will handle that 👍

Proposed resolution

1. Add validation constraints.
2. Mark FullyValidatable.
3. Use #config_target in config forms wherever possible

Remaining tasks

Review.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

N/A