chore: upgrade Node.js to latest LTS (24) across the repo

[josephfusco]

Aligns the repository's CI matrices, default Node version, .nvmrc, and package engines to the supported Node LTS line (drops EOL 18/20, defaults to 24, keeps 22 in CI for the active LTS).

Closes #2369

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[josephfusco]

Closing in favor of a phased approach.

Existing customers are running production deployments on the Node versions this PR would force off (Node 20 in particular). Bumping engines.node to >=22 across all packages would either (a) emit install warnings for those customers, (b) hard-fail their engine-strict installs, or (c) force a Node runtime upgrade in lockstep with a routine package upgrade.

Splitting into two phases:

• Phase 1 (now) — ci: drop Node 18 from CI matrix, add Node 24 #2371 — CI matrix only. Adds Node 24, keeps Node 20, drops Node 18 (EOL March 2025). No engines change, no major version bump, no customer impact.
• Phase 2 (later) — engines.node >=22 + major version bump (4.0.0 for core/cli, matching the v3.0.0 precedent that dropped Node 16) + migration doc. Will be scheduled once customer adoption of Node 22+ has organically moved further along.

The issue (#2369) will be updated to reflect the phased plan.