Prune dead servers from `ecosystem.json`

[cjpatton]

Some servers appear to be down:

[master][~/github.com/cloudflare/roughtime]$ go run ./cmd/getroughtime -config ecosystem.json
skipped Chainpoint-Roughtime: no reply
Cloudflare-Roughtime: 2023-12-15 08:12:05.05 -0800 PST ±1s (in 38ms)
skipped Google-Sandbox-Roughtime: no reply
int08h-Roughtime: 2023-12-15 08:12:08.117072 -0800 PST ±1s (in 89ms)
skipped ticktock: no reply
time.txryan.com: 2023-12-15 08:12:11.225653 -0800 PST ±1s (in 110ms)
delta: 12ms

If they're permanently dead then they should be removed from the list. However we should try to reach out to the maintainers: It may be that they implement a version of Roughtime that we're not compatible with or they have moved their server to a different address.

[cjpatton]

Per discussion with @lukevalenta: Add which version each server is known to support.