docs: clarify Linaria 7 requirements

[Anber]

Small release-train doc tweak for Linaria 7:

• Mention Node.js 20+ requirement (aligned with WyW 1.x) in docs/MIGRATION_GUIDE.md.
• Add the same note to the 7.0.0 major changeset bump-wyw-1-0-x.md so it shows up in release notes.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 701602d

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes changesets to release 16 packages

Name	Type
@linaria/atomic	Major
@linaria/babel-plugin-interop	Major
@linaria/core	Major
@linaria/postcss-linaria	Major
@linaria/react	Major
@linaria/server	Major
@linaria/stylelint	Major
@linaria/stylelint-config-standard-linaria	Major
@linaria/testkit	Major
linaria	Major
linaria-website	Patch
vpssr-linaria-solid-example	Patch
esbuild-example	Patch
rollup-example	Patch
vite-example	Patch
webpack5-example	Patch

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