chore(deps): update php_serializer requirement from ~> 0.9.0 to ~> 2.0.0

Updates the requirements on [php_serializer](https://github.com/zloyrusskiy/php_serializer) to permit the latest version.
- [Release notes](https://github.com/zloyrusskiy/php_serializer/releases)
- [Commits](https://github.com/zloyrusskiy/php_serializer/commits/v2.0)

apps/content/mix.exs

@@ -57,7 +57,7 @@ defmodule Legendary.Content.MixProject do
       {:phoenix_html_sanitizer, "~> 1.0.0"},
       {:phoenix_live_reload, "~> 1.2", only: :dev},
       {:phoenix_live_dashboard, "~> 0.4.0"},
-      {:php_serializer, "~> 0.9.0"},
+      {:php_serializer, "~> 2.0.0"},
       {:plug_cowboy, "~> 2.0"},
       {:sitemap, "~> 1.1"},
       {:slugger, "~> 0.3"},