Update sobelow ignore list

Sobelow was throwing an error because we are no longer calling
:put_secure_browser_headers in router.ex, however, we have wrapped
it with ExplorerWeb.CSPHeader and are still calling it.

In the process it became apparent that having Config.CSRF in the
ignore list wasn't actually doing anything so it has been removed.

apps/explorer_web/.sobelow-conf

@@ -5,5 +5,5 @@
   router: "lib/explorer_web/router.ex",
   exit: "low",
   format: "compact",
-  ignore: ["Config.CSRF"]
+  ignore: ["Config.Headers"]
 ]