[docs] Make Go dynamic for API docs gen

[artberger]

Reference workflow was broken with:

2026-04-09T12:08:33.327Z    ERROR    crd-ref-docs    Failed to process source directory    {"error": "failed to   
  find API types in directory /home/runner/work/website/website/website/../agentgateway/controller/api/v1alpha1/agentgateway/:load packages in root                               
  \"/home/runner/work/website/website/agentgateway/controller/api/v1alpha1/agentgateway\": err: exit status 1: stderr: go: ../../../../go.mod requires go >= 1.25.8 (running go   
  1.25.7; GOTOOLCHAIN=local)\n"} 

The workflow used to hardcode Go to 1.25.7, but agentgateway's main branch now requires Go 1.25.8. This fix is to dynamically read the Go version from agentgateway's go.mod instead of hardcoding it