chore: improve workflows

[luc122c]

Summary

This PR includes various improvements to the workflows.

• use ubuntu-slim where possible. It's a lightweight runner that runs in a container instead of a full VM. See the blog post and readme for more info.
• use background and parallel where possible. This blog post details how parallel and background and wait can be used to do async work.
• drop node 20 in workflows. Node 20 is EOL and is deprecated in actions. I've replaced it with LTS where possible.
• drop node 25 in workflows. Node 25 is an odd-numbered release and is EOL. I've replaced it with 26 which is current.
• refactor close-incorrect-issue.yml. This was doing a full npm ci to run the node script. which depended on octokit. I've used an agent to refactor it to a github-script like ai-slop.yml

Please follow this checklist to avoid unnecessary back and forth (click to expand)

• ⚠️ If there are Breaking change (a fix or feature that alters existing functionality in a way that could cause issues) I have called them out
• 🧠 I have disclosed any use of LLMs/AI in this contribution and reviewed all generated content.
  I understand that I am responsible for and able to explain every line of code I submit.
• 🔍 Any UI changes adhere to visual style of this project.
• 🛠️ I have self-reviewed and self-tested my code to ensure it works as expected.
• 📝 I have commented my code, especially in hard-to-understand areas (e.g., using JSDoc for methods).
• 🤖 I added or updated automated tests where appropriate.
• 📄 Documentation updates are included (if applicable).
• 🧰 Dependency updates are listed and explained.
• ⚠️ CI passes and is green.

[github-actions]

Thanks for the PR! If anyone would like to help with testing, run: npx kuma-pr luc122c:chore/improve-workflows (requires Node.js and Docker)

[github-actions]

Hello and thanks for lending a paw to Uptime Kuma! 🐻👋
As this is your first contribution, please be sure to check out our Pull Request guidelines.
In particular: - Mark your PR as Draft while you’re still making changes - Mark it as Ready for review once it’s fully ready
If you have any design or process questions, feel free to ask them right here in this pull request - unclear documentation is a bug too.