ravenports -- use etag and If-None-Match

[jrmarino]

the repology.json file for ravenports is static between updates. There could be days between updates.
The size of repology.json is not insignificant. presenting the etag and only downloading upon change would be faster and save bandwidth. Etags are stable on github.

Just a thought.

[AMDmi3]

Ravenports takes 1 sec to fetch and 3 secs to parse, there's nothing to optimize here, and we don't have sources affected by this optimization which are considerably heavier. For most sources if-modified-since which we already support works fine. So I don't plan to change anything right now, but we'll switch to etag during updater rust rewrite.