Go v2: v1→v2 breaking-change list & migration guide

[tvaron3]

Part of the Cosmos DB Go SDK v2 (azcosmos/v2) effort, delivering the Go v2 data plane over an FFI/cgo binding to the shared Rust Cosmos driver (decision: #27238, ADR sdk/data/azcosmos/docs/adr/0001-go-v2-uses-ffi.md).

Split out of #27306, which owns the v2 API surface itself. This issue owns the customer-facing delta between v1 and v2 — a separate deliverable with a different audience and a different completion point (it can only be finished once the v2 surface stops moving).

Scope

• Classify the v1 exported surface: kept as-is / changed / removed / new.
• Write the v1→v2 breaking-change list into the 2.0.0-beta.1 section of sdk/data/azcosmos/CHANGELOG.md.
• Write sdk/data/azcosmos/migrationguide.md with side-by-side v1/v2 samples, following the sdk/messaging/azeventhubs/migrationguide.md precedent.
• Record the cgo fallout for migrating customers: the CGO_ENABLED=1 requirement, the cross-compilation story, and the fact that v1's wasm support (cosmos_default_http_client_dialer_wasm.go) cannot carry to v2.

Deliverables

• CHANGELOG.md breaking-change list under 2.0.0-beta.1.
• sdk/data/azcosmos/migrationguide.md.

Definition of done

• Every removed or changed v1 export has either a documented v2 replacement or a stated rationale.
• Migration guide reviewed by the crew.

Notes

• The v1 exported-surface inventory must be captured before the v2 reset PR deletes the v1 sources; that inventory is the input to this issue.
• The v2 reset PR leaves a migrationguide.md stub for this issue to fill.

Depends on: #27306 (the v2 surface must be stable enough to write against).