Update code-generator to 1.30.1

[xrstf]

While working on #293, I noticed I had to bump the code-generator as well, which blew up that PR more than it should. So this PR extracts just the code-generator update.

The code generator was significantly outdated and was updated to match the current set of Kube dependencies (i.e. 1.30.x). Notably:

• The entire GOPATH cloning logic was removed; all generators are now compatible with Go modules and there is no need anymore to ensure a GOPATH-like setup (I ran everything locally in ~/projects/prow).
• The GOPATH magic mentions bazel peculiarities as one of the reasons for the complexity. I could not immediate see where exactly Bazel is currently used in Prow, so I simply chose to ignore it and see which wall I would hit. 😁 Bazel support seems to have been removed 2 years ago via Bazel build/test instructions are replaced with make rules kubernetes/test-infra#26039.
• Not working in a mirrored version also fixed the bindata generation: That step never actually copied the files back to the repository and so we never noticed that someone manually fixed the %v to be %w, because go-bindata 3.1.3 (latest) still outputs %v.

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[xrstf]

/test all

[petr-muller]

/cc