backupccl: zone config for system tables not included in backup are not restored

If a backup is taken on a cluster where there is a custom GC TTL applied to a system table that is not included in backups, the custom TTL will not be present when the backup is restored. This can be specially problematic as it makes upgrade migrations such as the one introduced in #100141 ineffective; when the database is restored, the custom TTL will no longer apply (the corresponding row in `system.zones` will not exist) and that could lead to degraded observability performance for reasons that might be hard to diagnose.

### Reproduction

This behavior was first identified by running the `backup/mixed-version` roachtest in #100755. However, it can be readily observed by running `TestFullClusterBackup` and disabling the `SkipZoneConfigBootstrap` testing knob in [1].

[1] https://github.com/cockroachdb/cockroach/blob/7d219406b9c0fc5079403840d55ba40f721512f9/pkg/ccl/backupccl/utils_test.go#L105-L107

Jira issue: CRDB-26652