[Go] Fix pubsubio import of removed genproto package

[DiezNobach]

pubsubio imported google.golang.org/genproto/googleapis/pubsub/v1 just to use
PubsubMessage as a plain proto struct. Recent genproto releases dropped that
package, breaking builds for any module needing a newer genproto. The actual
read/write logic never used a gRPC client from it — it delegates to Dataflow's
native pubsub_read/write external transform.

This swaps the import for cloud.google.com/go/pubsub/v2/apiv1/pubsubpb, which is
wire-compatible (same field numbers) — no behavior change.

• fixes [Bug]: pubsubio imports google.golang.org/genproto/googleapis/pubsub/v1, which was removed from current genproto releasess #40018
• Updated CHANGES.md.

[github-actions]

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