v1.33.0 should be retracted (using the retract directive)

[menghanl]

(This issue is file to track the problem described in #4366.)

Release 1.33.0 was published, and was removed shortly after because it has problems (#3945 (comment)). But the version also shows up in https://pkg.go.dev/google.golang.org/grpc@v1.33.0 even after it's deleted, and the users can still depend on it (by specifying it in go.mod) by mistake.

In go 1.16, a new directive was introduced to help solve this problem: https://golang.org/ref/mod#go-mod-file-retract

But it cannot be used at the time this issue is created, because go versions older than 1.16 are still supported.

[dfawley]

I don't see the need for this for this particular instance. By the time we can add it (~1yr), v1.33.0 will be so old nobody would have any reason to reference it. It's already 4 releases old, which is typically older than we're willing to maintain via patch releases.

If this kind of thing were to happen again, it's nice to know there is a friendly way of dealing with it.