Pin default AppKit version to 0.20.3 (#4763)

Bump appkitDefaultVersion from template-v0.14.1 to template-v0.20.2 so
that `databricks apps init` scaffolds with AppKit 0.20.2 by default.

## Changes
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https://dev-evals-monitor-6177827686947384.gcp.databricksapps.com/

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Author: keugenek

cmd/apps/init.go

@@ -37,7 +37,7 @@ const (
 	appkitTemplateDir    = "template"
 	appkitDefaultBranch  = "main"
 	appkitTemplateTagPfx = "template-v"
-	appkitDefaultVersion = "template-v0.14.1"
+	appkitDefaultVersion = "template-v0.20.3"
 	defaultProfile       = "DEFAULT"
 )