build: update README for deprecation notice

[sofisl]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

Description

Please provide a detailed description for the change.
As much as possible, please try to keep changes separate by purpose. For example, try not to make a one-line bug fix in a feature request, or add an irrelevant README change to a bug fix.

Impact

What's the impact of this change?

Testing

Have you added unit and integration tests if necessary?
Were any tests changed? Are any breaking changes necessary?

Additional Information

Any additional details that we should be aware of?

Checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease
• Appropriate docs were updated
• Appropriate comments were added, particularly in complex areas or places that require background
• No new warnings or issues will be generated from this change

Fixes #issue_number_goes_here 🦕

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml