workbench: diff-suppress server-injected version-description metadata

[jeffreylwang]

The Workbench backend surfaces a system-owned instance metadata key version-description (a human-readable image description derived from the VmImage release configs, e.g. Debian 11, Python 3.10). Users can never set this key, so the google_workbench_instance provider must diff-suppress it; otherwise every Terraform-managed Workbench instance gets a permadiff (same class of issue as new-proxy-agent-enabled, #17310).

This adds version-description to WorkbenchInstanceProvidedMetadata (purely server-owned, always injected, not EUC-specific).

Context (Google-internal): b/539638368.

workbench: suppressed a diff on `google_workbench_instance` for the server-injected `version-description` metadata key

[github-actions]

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[jeffreylwang]

@bcreddy-gcp could you please take a look / review this? This adds versionDescription to WorkbenchInstanceProvidedMetadata (the Workbench server-injected metadata diff-suppress list) to prevent a permadiff on google_workbench_instance, matching the precedent from #17310. You're listed as the point of contact for the Workbench metadata guard. Thanks!

[jeffreylwang]

Renamed the metadata key from versionDescription to version-description for consistency with the other kebab-case Workbench metadata keys (the PR diff now adds version-description).