Add legacy 6.4 version to list

Author: MaedahBatool

docs/legacy.mdx

@@ -4,6 +4,7 @@
 
 <Accordion title="Sourcegraph 6.X">
 
+  - [6.4](https://6.4.sourcegraph.com)
   - [6.3](https://6.3.sourcegraph.com)
   - [6.2](https://6.2.sourcegraph.com)
   - [6.1](https://6.1.sourcegraph.com)