Add composite action for version control of Node and pnpm

[mattallford]

• Adds a composite action at .github/composite/versions/action.yml that centralises the standard Node (24) and PNPM (10.13.1) versions used by reusable workflows in this repo.
• Updates both microsite-deployment-branch.yml and microsite-deployment-full.yml to consume those versions from the composite action instead of hardcoding them.
• Removes the node_version workflow input (previously defaulting to 22) from both workflows, since consumers no longer choose the version.

Why?
Node and PNPM versions were drifting between workflows (and callers could pass their own Node version through the input). Centralising them in a single composite action means every consumer of these reusable workflows gets the same toolchain, and bumping versions is a one-line change in one file.