Modernize Node and pnpm setup across GitHub Actions workflows

[tbinna]

Problem

The GitHub Actions workflows still use pnpm/action-setup@v6 followed by actions/setup-node@v5. Current CI logs contain warnings owned by those actions rather than by nx-forge:

• actions/setup-node@v5 emits Node DEP0040 for punycode and DEP0169 for url.parse().
• pnpm/action-setup@v6 emits a false-positive pnpm v10 installation-layout warning when its bootstrap pnpm self-updates to the version pinned by the repository.
• On Windows, pnpm/action-setup@v6 emits DEP0190 because its self-installer uses shell-based child-process execution.
• pnpm/action-setup now directs pnpm 11+ users to its successor, pnpm/setup.

PR #210 fixes the cache-write race introduced by its parallel CI jobs, but the repository-wide action setup should be modernized separately and consistently.

Scope

• Update every workflow that installs pnpm and Node, including PR CI, main CI, documentation, deployment, and release workflows.
• Prefer the supported pnpm/setup@v2 path, or document a justified supported alternative.
• Preserve the repository pnpm version declared in package.json.
• Preserve Node LTS coverage and frozen-lockfile enforcement.
• Define cache ownership so parallel jobs do not try to save the same cache key.
• Do not suppress deprecations or pin pnpm to an action-internal bootstrap version merely to hide warnings.

Acceptance criteria

• All GitHub Actions workflows use one consistent, supported Node and pnpm setup.
• The pinned pnpm version and Node LTS behavior are preserved.
• CI installs fail for a missing or stale lockfile.
• Concurrent jobs do not contend for the same cache write.
• The action-owned pnpm layout, DEP0190, DEP0040, and DEP0169 warnings no longer appear.
• PR CI passes on Ubuntu and Windows.
• Main, documentation, deployment, and release workflows retain their current behavior.

References

• v6.0.9: false-positive "v10 installation layout" warning when self-updating from bootstrap 11.7.0 (still reproduces with pnpm 11.19.0) pnpm/action-setup#281
• v6 regression: dest input breaks when path contains a space (Windows) pnpm/action-setup#261
• v5+: DeprecationWarning: The punycode module is deprecated actions/setup-node#1364
• v5 DeprecationWarning: url.parse() behavior is not standardized actions/setup-node#1370
• https://github.com/pnpm/setup
• fix(executors): launch Forge without shell interpretation #210