Add REST API deprecation notice to README

[nasudadada]

Add REST API deprecation notice to README

WHY are these changes introduced?

Related to #751 #745

According to Shopify's announcement, the REST Admin API will become legacy as of October 1, 2024. This change adds necessary warnings to the README to inform users about the upcoming deprecation and migration requirements.

WHAT is this pull request doing?

• Adds deprecation notice to the Making API Calls section
• Adds deprecation notice to the Advanced Usage section
• Includes timeline information (February 2025 for public apps, April 2025 for custom apps)
• Adds link to Shopify's official migration guide
• Updates code examples with deprecation comments

Checklist

• [ ] I have updated the CHANGELOG (if applicable)
• I have followed the Shopify Python guide

[nasudadada]

I have signed the CLA

[sle-c]

thank you for adding this

[nasudadada]

@sle-c
Thank you for the review and approval!

I notice the pre-commit check is failing and blocking the merge. I'll create a separate issue to address this.

[nasudadada]

@sle-c The CI check is failing, but I believe this is due to the CI issue that has already been fixed in the #755.
What would be the preferred way to re-trigger the CI checks in this case?

Options I'm considering:

1. Adding an empty commit
2. Merging upstream changes
3. Waiting for maintainer's intervention

Please advise on the best approach for this project.

[sle-c]

Hi @nasudadada ,

In this case I'll just merge the PR because it's a readme change. Thank you for your contribution.

Si