Add RFC to deprecate generating new projects with yarn

text/9999-deprecate-new-project-yarn-support.md

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+---
+stage: accepted
+start-date:
+release-date:
+release-versions:
+teams: # delete teams that aren't relevant
+  - cli
+  - data
+  - framework
+  - learning
+  - steering
+  - typescript
+prs:
+  accepted: # update this to the PR that you propose your RFC in
+project-link:
+---
+
+<!---
+Directions for above:
+
+stage: Leave as is
+start-date: Fill in with today's date, 2032-12-01T00:00:00.000Z
+release-date: Leave as is
+release-versions: Leave as is
+teams: Include only the [team(s)](README.md#relevant-teams) for which this RFC applies
+prs:
+  accepted: Fill this in with the URL for the Proposal RFC PR
+project-link: Leave as is
+-->
+
+# Deprecate `--yarn` support for `ember new`
+
+## Summary
+
+Deprecate the `--yarn` flat for `ember new`. We believe newly generated projects
+will have a better experience using `npm` or `pnpm` as the package manager.
+
+## Motivation
+
+`ember-cli` currently only supports `yarn@1.x`. The latest version of `yarn` is `4.x`. 
+Most contributors to the project are using `npm` or `pnpm` as their package manager and are not motivated 
+to update `yarn` support. An [open issue to update support](https://github.com/ember-cli/ember-cli/issues/10339) 
+has been open for over nearly 18 months without being addressed. 
+
+We know that modern `yarn` can be used successfully with Ember projects, but it requires
+disabling Yarn's `PnP` feature. Generating a new project also requires some gymnastics to
+ensure that the correct version of `yarn` is installed.
+
+We believe that the best experience for new projects is to use `npm` or `pnpm` as the package manager.
+
+## Transition Path
+
+Deprecate the `--yarn` flag to `ember new` and `ember addon`. Remove it from help and documentation.
+
+For now, we will continue to support existing projects that use `yarn` as their package manager.
+
+
+## How We Teach This
+
+Remove any documentation of generating projects with `--yarn`. 
+
+## Drawbacks
+
+
+## Alternatives
+
+We could update support for `yarn` in the ecosystem, but this would likely only happen if an interested
+party were to volunteer. 
+
+## Unresolved questions
+
+> Optional, but suggested for first drafts. What parts of the design are still
+TBD?