backupccl: restoring a database does not insert an entry for it's public schema in system.namespace

[RichardJCai]

See:
https://github.com/RichardJCai/cockroach/blob/backup_test_missing_public_schema_namespace_entry/pkg/ccl/backupccl/restore_old_version_public_schema_test.go

Notice theres no 52 0 public 29 entry.

Output:

0 0 system 1
1 29 descriptor 3
1 29 users 4
1 29 zones 5
1 29 settings 6
1 29 tenants 8
1 29 lease 11
1 29 eventlog 12
1 29 rangelog 13
1 29 ui 14
1 29 jobs 15
1 29 web_sessions 19
1 29 table_statistics 20
1 29 locations 21
1 29 role_members 23
1 29 comments 24
1 29 replication_constraint_stats 25
1 29 replication_critical_localities 26
1 29 replication_stats 27
1 29 reports_meta 28
51 0 public 29
50 0 public 29
1 0 public 29
1 29 namespace 30
1 29 protected_ts_meta 31
1 29 protected_ts_records 32
1 29 role_options 33
1 29 statement_bundle_chunks 34
1 29 statement_diagnostics_requests 35
1 29 statement_diagnostics 36
1 29 scheduled_jobs 37
1 29 sqlliveness 39
1 29 migrations 40
1 29 join_tokens 41
1 29 statement_statistics 42
1 29 transaction_statistics 43
1 29 database_role_settings 44
1 29 tenant_usage 45
1 29 sql_instances 46
1 29 span_configurations 47
0 0 defaultdb 50
0 0 postgres 51
0 0 d 52
52 0 s 53
52 53 t 54
52 29 t 55

Context: #72662 (comment)

• actually insert a system.namespace entry on restore
• add this case to debug doctor

[blathers-crl]

cc @cockroachdb/bulk-io