microsite: update remaining newsletter links

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

docs/.release-notes-template.md

@@ -37,4 +37,4 @@ Below you can find a list of links and references to help you learn about and st
 - [Changelog](https://github.com/backstage/backstage/tree/master/docs/releases/v1.1.0-changelog.md)
 - Backstage [Demos](https://backstage.io/demos), [Blog](https://backstage.io/blog), [Roadmap](https://backstage.io/docs/overview/roadmap) and [Plugins](https://backstage.io/plugins)
 
-Sign up for our [newsletter](https://info.backstage.spotify.com/newsletter_subscribe) if you want to be informed about what is happening in the world of Backstage.
+Sign up for our [newsletter](https://spoti.fi/backstagenewsletter) if you want to be informed about what is happening in the world of Backstage.

docs/overview/support.md

@@ -15,7 +15,7 @@ description: Support and Community Details and Links
 - [Code of Conduct](https://github.com/backstage/backstage/blob/master/CODE_OF_CONDUCT.md) -
   This is how we roll.
 - [Blog](https://backstage.io/blog/) - Announcements and updates.
-- [Newsletter](https://info.backstage.spotify.com/newsletter_subscribe) - Subscribe to
+- [Newsletter](https://spoti.fi/backstagenewsletter) - Subscribe to
   our email newsletter.
 - Give us a star ⭐️ - If you are using Backstage or think it is an interesting
   project, we would love a star! ❤️