Remove dependency on go.dev API in determine-versions workflow job

## Summary

The `determine-versions` job in `.github/workflows/ubi8_multi_arch_image_build.yml` currently relies on a network call to `https://go.dev/dl/?mode=json` to discover previous supported Go versions for matrix builds.

## Problem

- The workflow has no error handling or retry logic for transient HTTP failures (5xx, timeouts), making scheduled builds potentially flaky.
- A hard dependency on the go.dev API means any outage or API change can break the build pipeline.

## Goal

Remove or replace the dependency on the go.dev API with a more reliable alternative (e.g., hardcoding supported versions, using the GitHub API for Go releases, or another stable source).

## References

- PR: https://github.com/konveyor/builder/pull/31
- Review comment: https://github.com/konveyor/builder/pull/31#discussion_r2990515551

/cc @kaovilai