Require Node.js 20 and update CI/docs

[coliff]

Bump the project's Node requirement to v20 and align tooling and docs. Updates package.json engines and Volta pin, .nvmrc, README, CONTRIBUTING, copilot-instructions.md, and the getting-started docs to require Node.js 20. Adjust CI workflows by adding concurrency groups to ossf-scorecard, spellcheck, and super-linter, and remove Node 18 from the test matrix so tests run on 20/22/24. These changes standardize the development environment on Node 20 LTS and reduce duplicated CI runs.

[gemini-code-assist]

Code Review

This pull request updates the project to require Node.js v20, aligning documentation and configuration files accordingly. The changes are generally sound and move the project to a modern LTS version of Node.js. However, I've identified a critical issue: the specified Node.js version 20.20.1 in both .nvmrc and package.json (for Volta) is not a valid Node.js release. This will break the development environment setup for anyone using nvm or volta. I've provided suggestions to correct this to a valid LTS version.

[Copilot]

Pull request overview

This PR raises the core HTMLHint project’s minimum supported Node.js version to 20+, updates version pins, aligns docs to the new requirement, and tweaks CI to reduce redundant workflow executions.

Changes:

• Update Node requirement and tooling pins (engines/Volta and .nvmrc) to Node 20.
• Refresh docs to state Node.js 20+ is required.
• Adjust CI: remove Node 18 from the test matrix and add workflow-level concurrency controls to reduce duplicate runs.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
website/src/content/docs/getting-started.mdx	Adds Node.js 20+ requirement statement to getting-started docs.
package.json	Bumps engines.node to >=20 and Volta pin to Node 20.20.1.
package-lock.json	Aligns lockfile top-level engines metadata with Node >=20.
README.md	Documents Node.js 20+ requirement for users.
.nvmrc	Pins nvm to Node 20.20.1 to match Volta.
.github/workflows/test.yml	Removes Node 18 from CI test matrix (now 20/22/24).
.github/workflows/super-linter.yml	Adds concurrency configuration to reduce duplicate linter runs.
.github/workflows/spellcheck.yml	Adds concurrency configuration for spellcheck runs.
.github/workflows/ossf-scorecard.yml	Adds concurrency configuration for scorecard runs.
.github/copilot-instructions.md	Updates Copilot guidance to reflect Node 20+ / Volta pin.
.github/CONTRIBUTING.md	Adds “Development setup” section documenting Node 20+ requirement.

[Copilot]

The concurrency group here differs between push and pull_request runs (PR uses the number; push uses the ref). That means a push-triggered run on a PR branch won’t be canceled when the pull_request-triggered run starts, so you can still get duplicate runs for the same branch/PR. Consider using a shared key like the head branch name for PRs (e.g., github.head_ref) and github.ref_name for pushes, or drop the push trigger if pull_request coverage is sufficient.

Suggested change

	group: super-linter-${{ github.event.pull_request.number || github.ref }}
	group: super-linter-${{ github.head_ref || github.ref_name }}