ci(workflows): make releaseversion input optional

Update the `build-image-template.yml` workflow to set `releaseversion`
as an optional input. This allows the workflow to run with the
default value when no specific OS release is provided.

Author: florian-asche

.github/workflows/build-image-template.yml

@@ -32,10 +32,10 @@ on:
         default: false
         description: "Enable rpi-imager.json snippet generation"
       releaseversion:
-        required: true
+        required: false
         type: string
         default: "trixie"
-        description: "Final image name"
+        description: "OS Release"
 
 jobs:
   build_images: