fix: Use valid Java 21 LTS version in mise config

The previous version string "25.0.1+8.0.LTS" was invalid and caused
CI failures. Java 21 is the current LTS version.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: pepicrft

.mise.toml

@@ -16,7 +16,7 @@ node = "24.11.1"
 pnpm = "latest"
 fnox = "latest"
 age = "latest"
-java = "25.0.1+8.0.LTS"  # LTS version
+java = "21"  # LTS version
 "npm:wrangler" = "latest"
 
 [tasks."docs:deploy-with-secrets"]