Generate `.pb.go` files with `any` instead of `interface{}`

[snprajwal]

Since Go 1.18, the any type is available to use in place of interface{}. The Go LSP (gopls) also raises a lint on instances of interface{} mentioning that it can be modernized to any (efaceany.go). Would we want to make protoc-gen-go use the any type going forward over the interface{} type? I'd be happy to contribute the PR for this.

[stapelberg]

Contributing a PR for this change is the easy part, verifying that we can roll it out without trouble within Google is the more time-consuming part.

I would recommend to just ignore this for now. Developers don’t have to modernize generated code, that’s on whoever generates the code (that would be us).

[puellanivis]

As well, linters should be ignoring generated code anyways, so it shouldn’t bother anything. (N.B. I said “shouldn’t” not “doesn’t.)