Add your stripe key to appdata

[danirabbit]

In the new version of AppCenter Dashboard, we're no longer modifying your appdata before publishing, so you'll need to add your stripe key to appdata.xml under the key x-appcenter-stripe to continue to monetize your app in elementary OS 6

For your convenience, your public stripe key is: <value key="x-appcenter-stripe">pk_live_5jQXnM9vRslDM8gVpuV1S0b9</value>