ci: pin upgrade test to @tailwindcss/[email protected]

because the 4.1.5 upgrade tool introduced a dependency on tailwind
that I can't figure out how to resolve.

Gah, this entire experience has been entirely frustrating as a
downstream maintainer, FML

Author: flavorjones

lib/install/upgrade_tailwindcss.rb

@@ -46,7 +46,7 @@
 
 if system("npx --version")
   say "Running the upstream Tailwind CSS upgrader"
-  command = Shellwords.join(["npx", "@tailwindcss/upgrade", "--force", "--config", TAILWIND_CONFIG_PATH.to_s])
+  command = Shellwords.join(["npx", "@tailwindcss/upgrade@4.1.4", "--force", "--config", TAILWIND_CONFIG_PATH.to_s])
   success = run(command, abort_on_failure: false)
   unless success
     say "The upgrade tool failed!", :red