Use date instead cursor for GitHub backend

[Zlopez]

Description

After investigation of #1016 I found out, that the cursor is unreliable. It will be better to use date when the last version was retrieved as a reference point.

Requirements

• Remove latest_version_cursor field from project
• Change GitHub backend to use date instead of cursor when retrieving new versions

How this will make Anitya better

This will allow us to have Github ratelimit optimalisation without the unreliability of cursor.

[Zlopez]

After some investigation, I found out, that this isn't possible to do with GraphQL API. It doesn't allow you to limit the query by creation date of the tag.