[Hyperdrive] Correctly referencing HYPERDRIVE env variable

[Oxyjun]

Summary

closes #21920

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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PR Change Summary

Updated the Hyperdrive documentation to correctly reference the HYPERDRIVE environment variable for database connection parameters.

• Corrected the database connection parameters to use the HYPERDRIVE environment variable.
• Removed outdated references to DB_HOST, DB_USER, DB_PASSWORD, DB_NAME, and DB_PORT.

Modified Files

• src/content/docs/hyperdrive/get-started.mdx

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[github-actions]

Preview URL: https://a5180ad5.preview.developers.cloudflare.com
Preview Branch URL: https://jun-hyperdrive-env-variable.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/hyperdrive/get-started/	https://jun-hyperdrive-env-variable.preview.developers.cloudflare.com/hyperdrive/get-started/