docs: require Node 26 for local setup

[raisedadead]

freeCodeCamp/freeCodeCamp#70714 moves freeCodeCamp/freeCodeCamp to Node 26 (.nvmrc, engines.node: ">=26"). Two pages still require Node 24:

• src/content/docs/how-to-setup-freecodecamp-locally.mdx: the prerequisites table lists Node.js 24.x, "The Active LTS version".
• src/content/docs/troubleshooting-development-issues.mdx: "Your operating system must support Node.js 24."

Proposal, when freeCodeCamp/freeCodeCamp#70714 merges:

• Change both pages to Node.js 26.
• Keep "Active LTS" only if Node 26 is Active LTS at that time. Node 26 enters LTS on 2026-10-28.

The docs site's own Node version (how-to-work-on-the-docs-site.mdx, .devcontainer, CI) is out of scope.