Bump `sigs.k8s.io/gateway-api`

Nobody has claimed this yet.

## Summary

`go.mod` pins:

```
sigs.k8s.io/gateway-api v0.8.1
```

That is Gateway API v0.8.1 (August 2023), released before the `v1` (GA) API surface
existed. The rest of the project is ahead of it.

The pin has stuck because our generated policy CRD types embed gateway-api types
directly (`controller/gen/apis/policy/v1alpha1`, `controller/gen/apis/policy/v1beta3`),
and some of those types have moved or been removed upstream, so a naive bump does not
compile.

This issue is to scope the bump. The research below should be done and written up
before any code change.

## Research needed

**1. Which symbols do we actually consume, and what happened to each?**
Enumerate the gateway-api symbols referenced from `controller/gen/apis/policy/*` and,
for each, determine whether the current release still provides it, provides it under a
new name, or dropped it.

**2. What is the blast radius in our tree?**
Run the bump on a scratch copy and collect the full list of compile errors. Confirm
whether breakage is confined to the generated policy types or reaches hand-written
packages (`pkg/`, `cli/`, `controller/api`, `controller/k8s`, `controller/webhook`).

**3. Which binaries are affected?**
Establish which `main` packages link gateway-api today and via which import paths, and
whether any of them use it at runtime.

**4. Codegen**
Confirm `bin/update-codegen.sh` (or the `justfile` target) regenerates cleanly against
the new version.

## Why it matters

Blocks using typed gateway-api clients anywhere in the Go tree; see the
companion issue https://github.com/linkerd/linkerd2/issues/15618

Start with go.mod and the generated policy types under controller/gen/apis/policy/*, then compare their referenced gateway-api symbols with the target release. Run the dependency bump in a scratch copy and record compile errors, affected main packages, and runtime imports. Confirm bin/update-codegen.sh or the justfile target regenerates cleanly; done means this scope and impact write-up is complete.

Written by the indexing model from the issue text.