build: update README for deprecation notice

[GautamSharda]

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Description

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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

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Testing

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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:

• .kokoro/common.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node18/common.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/lint.sh - .kokoro files are templated and should be updated in synthtool
• .github/CODEOWNERS - CODEOWNERS should instead be modified via the codeowner_team property in .repo-metadata.json
• .github/workflows/ci.yaml - .github/workflows/ci.yaml (GitHub Actions) should be updated in synthtool
• 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