gcp: migrate Pub/Sub input and output to the pubsub/v2 SDK

[highlyavailable]

Google's Cloud Go SDK moved Pub/Sub to a v2 package, so this migrates the gcp_pubsub input and output over to it.

The main issue is that v2 splits the data clients from admin operations. Topic and subscription existence checks, IAM, and CreateSubscription aren't on Publisher/Subscriber anymore, so those now go through TopicAdminClient/SubscriptionAdminClient with a codes.NotFound check.

One behaviour change worth calling out: v2 dropped synchronous pull, so the input's sync field doesn't map to anything now. I left it in and marked it deprecated rather than removing it since this is a Stable connector, and the reader logs a warning if someone sets it. Can rip it out instead if you'd prefer.

Ran the unit tests and the full gcp_pubsub integration suite against the emulator, all green.

Happy to take feedback and iterate on any of this.

Fixes #4041

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[josephwoodward]

Thank you for this @highlyavailable!

[highlyavailable]

Quick heads up on the most recent commit (c4528b6): two of the failing golangci-lint findings were in internal/impl/aws/s3/bench/read/redpanda-connect/main.go, which is unrelated to the gcp_pubsub migration in this PR. They look like new modernize rules picking up pre-existing patterns (a sync.WaitGroup use that can use wg.Go, and an if/assign that can use max). I included the two-line fix here just to keep this PR's CI green, but if you'd rather see those land in their own PR on main I'm happy to split them out.