gitignore: add .env.testing to .gitignore

[levintoo]

Why this PR?

• I always add .env.testing to .gitignore on all my projects — it’s a small thing that just makes life easier.
• Since issues and discussions are disabled in the starter kits, I thought a simple PR to all three would be the best way to suggest it.
• Laravel automatically picks up .env.testing to keep testing configs separate.
• Adding it to .gitignore helps avoid accidentally committing sensitive or local test environment details.
• Honestly, not sure why this isn’t already included by default.