tests: Fix webhook.py platform-check

[ParkMyCar]

This PR fixes the webhook.py platform-check.

In #30821 I failed to consider the scenario of when a platform check runs on a version < v0.128.0 and we create the webhook source with the CREATE SOURCE ... FROM WEBHOOK, and then later check with a version >= v0.128.0, like our Nightly tests do.

This PR changes the check to only use the new CREATE TABLE ... FROM WEBHOOK syntax if we're on a version >= v0.128.0

Motivation

Fix Nightly

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[bkirwi]

Kicked off the relevant nightlies... looks like they're still failing.

[ParkMyCar]

Kicked off the relevant nightlies... looks like they're still failing.

Yeah 🤦 really taking a moment to think through it, I'm pretty sure what I initially tried to do with this check just isn't possible. After the second iteration the tests were still failing because when the initialize step is run in a version <0.128.0 which means the table just wouldn't be created.

Updated this to move the CREATE TABLE ... FROM WEBHOOK to its own check entirely with a _can_run(...). Third time is the charm!