Fetch gap release data from GitHub rather than gap-system.org

[Joseph-Edwards]

The macOS runners using this action have recently started failing (see, for example, semigroups/Semigroups#1087 or https://github.com/Joseph-Edwards/setup-gap/actions/runs/21633236857/job/62351347716). This seems to be because wget https://www.gap-system.org/releases.json is timing out after DNS succeeds but before an HTTPS connection can be established.

I'm not entirely sure why this has started happening, but it seems like something is blocking the runner's access to gap-system.org.

This PR address this issue by replacing https://www.gap-system.org/releases.json with it's "source" file https://raw.githubusercontent.com/gap-system/GapWWW/master/releases.json.

[Joseph-Edwards]

The failures in the windows jobs seem sporadic and network related

[stertooy]

This would also prevent server problems for the gap website impacting CI jobs (a few days ago I couldn't reach gap-system.org at all, and was redirected to a university error page instead). That may/would have impacted CI jobs as well, though I didn't run any myself during the downtime.

I'm not sure doing a wget to https://raw.githubusercontent.com is ideal, since the rate limits seem to be unknown (discussion on SO here). I don't immediately have a better solution, though, so maybe we should just try this and see how it goes?