use /api/js endpoint as new primary endpoint for server list fetching

[redradrat]

Switched to using https://www.speedtest.net/api/js/servers?engine=js&limit=10 as primary server list request.

This fixes #50.

[r3inbowari]

Thank you

As mentioned in #50. With time goes by, more and more new speedtest hosts are deployed with higher versions, so it is difficult to obtain close-range and compatible servers in some areas.

This api is obviously better than the original one. But I think the current version is not suitable for using it as the primary endpoint in this time due to some possible compatibility issues.

Because most of the server versions fetched by this api are newer than speedtest-go client, it will cause ping, upload, download to be abnormal when using these new servers. such as negative rate or unable to ping. Because the ping function has been implemented by websocket in the new official version of the server.

[showwin]

@redradrat Thank you for a great PR 👍
and
@r3inbowari Thank you for great support 🤝

I read an official document about How To Install & Submit Server, and I created testing server in my local based on OoklaServer Installation - Linux / Unix document.
Then it seems the latest server has a compatibility to our speedtest-go client.

So I accept @redradrat idea.