fix(deps): update module github.com/hashicorp/terraform-plugin-mux to v0.21.0

Author: renovate[bot]

go.mod

@@ -17,7 +17,7 @@ require (
 	github.com/hashicorp/terraform-plugin-framework-validators v0.18.0
 	github.com/hashicorp/terraform-plugin-go v0.28.0
 	github.com/hashicorp/terraform-plugin-log v0.9.0
-	github.com/hashicorp/terraform-plugin-mux v0.20.0
+	github.com/hashicorp/terraform-plugin-mux v0.21.0
 	github.com/hashicorp/terraform-plugin-sdk/v2 v2.37.0
 	github.com/hashicorp/terraform-plugin-testing v1.13.3
 	github.com/packethost/packngo v0.31.0