Updated version badge in readme.

The version badge in the readme was showing `v0.7.14`, which is a little out of date 😅.  The badge was tied to the now deprecated `@backstage/core` npm package.
Updated the badge to be based directly on the latest release in github.

Signed-off-by: Alex Crome <afscrome@users.noreply.github.com>

README.md

@@ -8,7 +8,7 @@
 [![Discord](https://img.shields.io/discord/687207715902193673)](https://discord.gg/EBHEGzX)
 ![Code style](https://img.shields.io/badge/code_style-prettier-ff69b4.svg)
 [![Codecov](https://img.shields.io/codecov/c/github/backstage/backstage)](https://codecov.io/gh/backstage/backstage)
-[![](https://img.shields.io/npm/v/@backstage/core?label=Version)](https://github.com/backstage/backstage/releases)
+[![](https://img.shields.io/github/v/release/backstage/backstage)](https://github.com/backstage/backstage/releases)
 
 ## What is Backstage?