Remove Flow Previewnet

[bthaile]

Small change

Flow Previewnet has been removed and not longer available.

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PR-Codex overview

This PR deprecates the Flow Previewnet and updates the chains/index.ts file to reflect this change.

Detailed summary

• Deprecated Flow Previewnet in chains/index.ts
• Updated export statements in chains/index.ts to reflect deprecation

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[changeset-bot]

🦋 Changeset detected

Latest commit: 1a9ef56

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
viem	Patch

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[bthaile]

Updated PR to revert deleting definition file, so only the /** @deprecated */ change to index is in this PR. Thanks