change `Repo.get(Activity, id)` => `Activity.get_by_id(id)`

lib/pleroma/gopher/server.ex

lib/pleroma/web/activity_pub/utils.ex

lib/pleroma/web/common_api/utils.ex

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

lib/pleroma/web/twitter_api/twitter_api.ex

lib/pleroma/web/twitter_api/twitter_api_controller.ex