Update fluent-plugin-kubernetes_metadata_filter to 2.11.1

To be compatible with BoundServiceAccountTokenVolume feature introduced in kubernetes 1.21.

More details:
* fluent/fluentd#3757
* https://kubernetes.io/docs/reference/access-authn-authz/service-accounts-admin/#bound-service-account-token-volume
* https://docs.aws.amazon.com/eks/latest/userguide/kubernetes-versions.html#kubernetes-1.21

Gemfile

@@ -6,7 +6,7 @@ gem 'fluentd', '1.14.6'
 gem 'fluent-plugin-concat', '2.5.0'
 gem 'fluent-plugin-detect-exceptions', '0.0.14'
 gem 'fluent-plugin-elasticsearch', '5.2.2'
-gem 'fluent-plugin-kubernetes_metadata_filter', '2.10.0'
+gem 'fluent-plugin-kubernetes_metadata_filter', '2.11.1'
 gem 'fluent-plugin-multi-format-parser', '1.0.0'
 gem 'fluent-plugin-prometheus', '2.0.2'
 gem 'fluent-plugin-systemd', '1.0.5'