Deprecate package

[devisscher]

What problem is this PR solving?

Summary of Changes:

The PR deprecates the @shopify/discount-app-components package. This involves adding a deprecation notice in the README.md and marking the package as deprecated in package.json. A changeset file .changeset/purple-pets-judge.md is introduced to document this change. The deprecation directs users to use the Polaris library and Admin UI Extensions instead.

Before going forward with deprecating this package the team will ensure:

1. Documentation Updates

   • Ensure the deprecation notice in README.md is prominently displayed at the top
   • Code examples showing how to replace common components with Polaris equivalents are found in: https://github.com/shopify/discounts-reference-app

2. Communication Plan

   • Draft an announcement for the Shopify developer blog/changelog
   • Prepare communication for the Shopify Partners Slack channel
   • Create a deprecation timeline with clear dates for:
     • Initial deprecation warning (current PR)
     • Final removal/archiving of the package

3. Technical Implementation

   • Update any CI/CD pipelines that might reference this package
   • Ensure npm deprecation notices are correctly configured

4. Quality Assurance

   • Test that the deprecation notice appears correctly on npm
   • Verify that the changeset properly documents the deprecation
   • Check that links to alternative solutions are valid and up-to-date

5. Post-Deprecation Follow-up

   • Plan for monitoring usage metrics to track adoption decline
   • Schedule check-ins to evaluate if timeline adjustments are needed
   • Create a process for handling critical security issues during deprecation period