Drop Ember 4.x support, add Ember 5 and 6 support

[uanjoze]

Summary

• Drops Ember <5.0 support entirely (including 4.x) and narrows peerDependencies.ember-source to ^5.0.0 || ^6.0.0.
• Adds ember-lts-5.4, ember-lts-5.8, ember-lts-5.12, ember-lts-6.4, ember-lts-6.8, ember-lts-6.12 scenarios to the ember-try matrix.
• Upgrades the addon and test-app toolchains (ember-cli, embroider v3 line, glint, ember-qunit, ember-data, @glimmer/component 2.x, babel, eslint config, commitlint/release-it/lefthook, etc.) to versions compatible with the new Ember range.
• Bumps the Node engine floor to >= 20 (required by Ember 6.x) and updates CI workflows accordingly.
• Fixes several build/CI breakages surfaced by the toolchain upgrade:
  • ember-cli-typescript is required to compile the test-app's .ts test files under the classic (non-Embroider) build pipeline.
  • ember-qunit@9 no longer auto-loads test modules from start() — now requires an explicit loadTests() call from ember-qunit/test-loader.
  • A stale transitive ember-tracked-storage-polyfill dependency (via tracked-built-ins) pinned an ancient ember-cli-htmlbars@5.7.2 that crashed ember-cli 7.x's addon model.
  • @tsconfig/ember's newer es2023 target isn't supported by TypeScript before 5.5 — pinned target/lib to es2022 in both tsconfig.json files.
  • @glimmer/component@1.x isn't compatible with newer Ember builds' component-manager registration — bumped to 2.x (only possible after dropping 4.x support).
  • @ember/test-waiters (transitive via ember-data) still imported the legacy ember AMD barrel module removed in Ember 7 — bumped to 4.x.
• Adds resolution-mode=highest to .npmrc and switches ember-try config from the deprecated usePnpm option to packageManager: 'pnpm'.

ember-release/ember-beta/ember-canary remain red: those bleeding-edge Ember builds restructured ember-source's dist/ output (dropping the flat dist/ember-template-compiler.js for a dist/dev + dist/prod split with an @ember/template-compiler package), which the current ember-cli-htmlbars/Embroider v3 toolchain can't locate. This is an Embroider v4 migration, out of scope here, and these 3 scenarios were already failing on master before this branch — left as a known-flaky early-warning signal.

Test plan

• pnpm lint passes for both ember-amount-input and test-app
• pnpm test:ember passes (9/9) on the default scenario (ember-source ~6.8)
• CI green: Tests, Floating Dependencies, commitlint, all ember-lts-5.x/6.x scenarios, embroider-safe/embroider-optimized, and typescript@5.0/5.1/5.2 type-checking
• ember-release/ember-beta/ember-canary/typescript@next confirmed pre-existing-flaky (already failing on master), not introduced by this PR

🤖 Generated with Claude Code