DEV: Use freeze_time_safe in more places (#25949)

Followup to 120a2f70a9ea3b08a39fc1fbb251f59ecf968cde,
uses new method to avoid time-based spec flakiness

spec/lib/backup_restore/system_interface_spec.rb

@@ -5,7 +5,7 @@ require_relative "shared_context_for_backup_restore"
 RSpec.describe BackupRestore::SystemInterface do
   subject(:system_interface) { BackupRestore::SystemInterface.new(logger) }
 
-  include_context "with shared stuff"
+  include_context "with shared backup restore context"
 
   describe "readonly mode" do
     after { Discourse::READONLY_KEYS.each { |key| Discourse.redis.del(key) } }