hide migrate to saas for demo mode (#25327)

Demo mode is run using the onpremise repo so we actually do want to keep certain UI functionality related to it (such as the version of Sentry). However, we don't need to show the Migrate to SaaS in demo mode.

static/app/components/footer.tsx

@@ -41,7 +41,7 @@ function Footer({className}: Props) {
         <FooterLink href="https://github.com/getsentry/sentry">
           {t('Contribute')}
         </FooterLink>
-        {config.isOnPremise && (
+        {config.isOnPremise && !config.demoMode && (
           <FooterLink href="/out/">{t('Migrate to SaaS')}</FooterLink>
         )}
       </Links>