guide: improve portability targets list

[grayside]

• Updates and links to Google Cloud hosting platform products
• Broadens other clouds into a bucket instead of specific references
• Simplifies language on portable code attributes

[github-actions[bot]]

🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-67/

[github-actions[bot]]

🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-67/

[github-actions[bot]]

🧨 Preview deployments removed.