pnpm version requirement cannot be met

A freshly scaffolded app cannot be installed/executed using pnpm, because the [version requirement of `>=9.0.0`](https://github.com/holochain-open-dev/templates/blob/main/templates/app/web-app/package.json.hbs#L20) cannot be met and such commands fail with

```sh
$ pnpm i
 ERR_PNPM_UNSUPPORTED_ENGINE  Unsupported environment (bad pnpm and/or Node.js version)

Your pnpm version is incompatible with "/Users/jost/Desktop/holochain/scaffolded-tryorama/peter".

Expected version: >=9.0.0
Got: 8.15.5

This is happening because the package's manifest has an engines.pnpm field specified.
To fix this issue, install the required pnpm version globally.

To install the latest version of pnpm, run "pnpm i -g pnpm".
To check your pnpm version, run "pnpm -v".
```

The latest version of pnpm to date is `8.15.6`.