Jenkinsfile: Use modern default

Jenkinsfile

@@ -9,7 +9,7 @@ node {
 
    parameters {
         string(name: 'board', defaultValue: "librem5r4", description: 'What board type to build for?')
-        string(name: 'dist', defaultValue: "buster+ci", description: 'What distribuition to build for?')
+        string(name: 'dist', defaultValue: "byzantium", description: 'What distribuition to build for?')
         string(name: 'build_type', defaultValue: "plain", description: 'What type of build is this?')
         string(name: 'publish', defaultValue: "false", description: 'Should the image get published?')
         string(name: 'kernel_url', defaultValue: "", description: 'The URL of the kernel')