build: update README for deprecation notice and delete all files except samples

[sofisl]

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Fixes # 🦕

[generated-files-bot[bot]]

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

• .kokoro/continuous/node12/common.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node12/lint.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node12/samples-test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node12/system-test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/continuous/node12/test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/lint.sh - .kokoro files are templated and should be updated in synthtool
• .kokoro/presubmit/windows/common.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/presubmit/windows/test.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/publish.sh - .kokoro files are templated and should be updated in synthtool
• .kokoro/release/docs-devsite.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/release/docs-devsite.sh - .kokoro files are templated and should be updated in synthtool
• .kokoro/release/docs.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/release/docs.sh - .kokoro files are templated and should be updated in synthtool
• .kokoro/release/publish.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/samples-test.sh - .kokoro files are templated and should be updated in synthtool
• .kokoro/system-test.sh - .kokoro files are templated and should be updated in synthtool
• .kokoro/test.bat - .kokoro files are templated and should be updated in synthtool
• .kokoro/test.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
• .github/generated-files-bot.yml - .github/generated-files-bot.(yml|yaml) 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