Fix embroider test scenarios

[dwickern]

embroider-safe and embroider-optimized fail with this error

  - broccoliBuilderErrorStack: Error: Cannot find module './vendor/ember/ember.js' from '/tmp/embroider/317b14/packages/test-app/node_modules/@embroider/synthesized-vendor'
    at Function.resolveSync [as sync] (/home/runner/work/ember-concurrency/ember-concurrency/node_modules/.pnpm/[email protected]/node_modules/resolve/lib/sync.js:111:15)
    at AppBuilder.impliedAddonAssets (/home/runner/work/ember-concurrency/ember-concurrency/node_modules/.pnpm/@[email protected]/node_modules/@embroider/core/src/app.js:219:55)
    at AppBuilder.impliedAssets (/home/runner/work/ember-concurrency/ember-concurrency/node_modules/.pnpm/@[email protected]/node_modules/@embroider/core/src/app.js:151:27)
    at AppBuilder.implicitScriptsAsset (/home/runner/work/ember-concurrency/ember-concurrency/node_modules/.pnpm/@[email protected]/node_modules/@embroider/core/src/app.js:374:40)
    at AppBuilder.insertEmberApp (/home/runner/work/ember-concurrency/ember-concurrency/node_modules/.pnpm/@[email protected]/node_modules/@embroider/core/src/app.js:340:29)
    at AppBuilder.prepareAsset (/home/runner/work/ember-concurrency/ember-concurrency/node_modules/.pnpm/@[email protected]/node_modules/@embroider/core/src/app.js:528:18)
    at AppBuilder.prepareAssets (/home/runner/work/ember-concurrency/ember-concurrency/node_modules/.pnpm/@[email protected]/node_modules/@embroider/core/src/app.js:538:18)
    at AppBuilder.updateAssets (/home/runner/work/ember-concurrency/ember-concurrency/node_modules/.pnpm/@[email protected]/node_modules/@embroider/core/src/app.js:605:27)
    at AppBuilder.build (/home/runner/work/ember-concurrency/ember-concurrency/node_modules/.pnpm/@[email protected]/node_modules/@embroider/core/src/app.js:686:38)
    at WaitForTrees.buildHook (/home/runner/work/ember-concurrency/ember-concurrency/node_modules/.pnpm/@[email protected]/node_modules/@embroider/core/src/build-stage.js:33:31)

This PR bumps the embroider version up from their defaults, which are

"@embroider/core": "^2.1.1",
"@embroider/webpack": "^2.1.1",
"@embroider/compat": "^2.1.1",

See CrowdStrike/ember-headless-form#550 - the same problem in a different addon

[machty]

Thank you!