Clarify API key language to note it needs to be added as the store secret in Foxy

[adamjudd]

As the Foxy language for the store API key has been updated to the "Store Secret", it's not immediately clear that that's where you need to set the FoxyShop API key when linking the two.

We should make a quick change to our note on the FoxyShop settings page that it's set in the "Store Secret" setting on the advanced settings page in Foxy.

[rijarobinson]

Update FoxyShop Version as well.