- Update to lockfile version 9 without updating dependencies
- Add npm corepack configuration for pnpm 9.15.6 (latest 9.x)
- Set lower bound for pnpm tp 9.15.4 (currently used by Netlify)
- Update dev container

- Update README, dev container and corepack reference
- No lockfile changes are necessary relative to pnpm 9

Remove pnpm version from  CI workflows to use the version specified in the root package.json's `packageManager` property.
This property is set by npm corepack.

Also update the pnpm setup actions and pin them to a fixed commit hash.