in_obsolete_plugins: support multi worker (#11)

If confiured using multi worker, `in_obsolete_plugins` causes following
config error.

### config
```
<system>
  workers 2
</system>

<source>
  @type obsolete_plugins
</source>
```

### error
```
2025-08-27 17:12:31 +0900 [error]: config error file="/home/watson/prj/sandbox/fluentd/fluent.conf" error_class=Fluent::ConfigError error="Plugin 'obsolete_plugins' does not support multi workers configuration (Fluent::Plugin::ObsoletePluginsInput)"
```

This PR will solve above config error.

Author: Watson1978

lib/fluent/plugin/in_obsolete_plugins.rb

@@ -15,6 +15,10 @@ class ObsoletePluginsInput < Fluent::Plugin::Input
       desc "Raise error if obsolete plugins are detected"
       config_param :raise_error, :bool, default: false
 
+      def multi_workers_ready?
+        true
+      end
+
       def configure(conf)
         super
 

test/plugin/test_filter_obsolete_plugins.rb

@@ -14,6 +14,15 @@ class ObsoletePluginsFilterTest < Test::Unit::TestCase
     Timecop.return
   end
 
+  test "multi worker" do
+    config = %[
+      obsolete_plugins_yml #{fixture_path("obsolete-plugins.yml")}
+    ]
+
+    d = create_driver(config)
+    assert_true d.instance.multi_workers_ready?
+  end
+
   sub_test_case "obsolete_plugins_yml" do
     CONFIG_YAML = %[
       obsolete_plugins_yml #{fixture_path("obsolete-plugins.yml")}

test/plugin/test_in_obsolete_plugins.rb

@@ -15,6 +15,15 @@ class ObsoletePluginsInputTest < Test::Unit::TestCase
     Timecop.return
   end
 
+  test "multi worker" do
+    config = %[
+      obsolete_plugins_yml #{fixture_path("obsolete-plugins.yml")}
+    ]
+
+    d = create_driver(config)
+    assert_true d.instance.multi_workers_ready?
+  end
+
   sub_test_case "plugins_json" do
     CONFIG_JSON = %[
       plugins_json #{fixture_path("plugins.json")}