Fix subpackage update workflow for 2.4 (#936)

Author: Cadair

.github/workflows/sub_package_update.yml

@@ -90,7 +90,7 @@ jobs:
           title: "Updates from the package template"
           labels: |
             No Changelog Entry Needed
-            Backport 2.3
+            Backport 2.4
           body: |
             This is an autogenerated PR, which will applies the latest changes from the [SunPy Package Template](https://github.com/sunpy/package-template).
             If this pull request has been opened as a draft there are conflicts which need fixing.