Swap to pnpm

[mansona]

With all the movement from @bertdeblock 's work I came here looking for a release-plan PR but we haven't set that up yet. I'm pretty sure that release-plan doesn't work with yarn classic so this PR is an attempt to swap over to pnpm.

It's a bit tricky because of the dual package-manager implementation in the smoke-test-app but we'll see if CI is happy 😂

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.49%. Comparing base (d3b56f1) to head (0cea39b).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1038   +/-   ##
=======================================
  Coverage   95.49%   95.49%           
=======================================
  Files          21       21           
  Lines         511      511           
=======================================
  Hits          488      488           
  Misses         23       23           

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[bertdeblock]

We can also set the packageManager field in the package.json file like so.
Helps avoiding stuff like this.
Also allows us to remove with: version: 9 everywhere in the CI file,
so the package.json field is the single source of truth for the pnpm version.

[mansona]

@bertdeblock yea I tried that but alas we can't do that because of the strange npm/yarn smoke tests. Corepack keeps looking up the directories even though it's not a workspace and it complains that you are in a folder that defines a different thing in the package.json 😞

You can see the failure here: https://github.com/ember-cli/ember-try/actions/runs/12430792672/job/34706945429

[bertdeblock]

Ah, too bad!