fix(deps): update module github.com/hashicorp/terraform-plugin-mux to v0.21.0 (#1349)

Co-authored-by: elastic-renovate-prod[bot] <174716857+elastic-renovate-prod[bot]@users.noreply.github.com>

Author: elastic-renovate-prod[bot]

go.mod

@@ -16,7 +16,7 @@ require (
 	github.com/hashicorp/terraform-plugin-framework-validators v0.18.0
 	github.com/hashicorp/terraform-plugin-go v0.29.0
 	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 v2.38.1
 	github.com/hashicorp/terraform-plugin-testing v1.13.3
 	github.com/mitchellh/mapstructure v1.5.1-0.20231216201459-8508981c8b6c

go.sum

@@ -617,8 +617,8 @@ github.com/hashicorp/terraform-plugin-go v0.29.0 h1:1nXKl/nSpaYIUBU1IG/EsDOX0vv+
 github.com/hashicorp/terraform-plugin-go v0.29.0/go.mod h1:vYZbIyvxyy0FWSmDHChCqKvI40cFTDGSb3D8D70i9GM=
 github.com/hashicorp/terraform-plugin-log v0.9.0 h1:i7hOA+vdAItN1/7UrfBqBwvYPQ9TFvymaRGZED3FCV0=
 github.com/hashicorp/terraform-plugin-log v0.9.0/go.mod h1:rKL8egZQ/eXSyDqzLUuwUYLVdlYeamldAHSxjUFADow=
-github.com/hashicorp/terraform-plugin-mux v0.20.0 h1:3QpBnI9uCuL0Yy2Rq/kR9cOdmOFNhw88A2GoZtk5aXM=
-github.com/hashicorp/terraform-plugin-mux v0.20.0/go.mod h1:wSIZwJjSYk86NOTX3fKUlThMT4EAV1XpBHz9SAvjQr4=
+github.com/hashicorp/terraform-plugin-mux v0.21.0 h1:QsEYnzSD2c3zT8zUrUGqaFGhV/Z8zRUlU7FY3ZPJFfw=
+github.com/hashicorp/terraform-plugin-mux v0.21.0/go.mod h1:Qpt8+6AD7NmL0DS7ASkN0EXpDQ2J/FnnIgeUr1tzr5A=
 github.com/hashicorp/terraform-plugin-sdk/v2 v2.38.1 h1:mlAq/OrMlg04IuJT7NpefI1wwtdpWudnEmjuQs04t/4=
 github.com/hashicorp/terraform-plugin-sdk/v2 v2.38.1/go.mod h1:GQhpKVvvuwzD79e8/NZ+xzj+ZpWovdPAe8nfV/skwNU=
 github.com/hashicorp/terraform-plugin-testing v1.13.3 h1:QLi/khB8Z0a5L54AfPrHukFpnwsGL8cwwswj4RZduCo=