Remove pnpm plugin in Gatsby example. (#7916)
### Description
Removes the `gatsby-plugin-pnpm` dependency because:
1) It doesn't look like it is needed. `turbo build` and `turbo dev`
still work.
2) It makes `npm` installation crash.
### Reviewer guide
Note that the only substantive changes are `web/package.json` and
`web/gatsby-config.ts`. The rest are from npm re-ordering the dependency
lists, package manager cleanup, and the lockfile updating.
Closes TURBO-2769
