Port Gitea lister to the new Lister API

Characteristics of the lister:

• instances: open list, default "try.gitea.io"
• API URL: instance_URL + "/api/v1/"
• query: "/repos/search?sort=id&order=asc&limit=%d&page=%d"
• state: none
• auth: none
• rate limiting: by instance -> generic handling
• pagination: 3 -> 50
• page format: JSON

Related to swh-scheduler#2442 (closed)

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Migrated from T2971 (view on Phabricator)