media/sticker: fix lottie mime type

It's always gunzipped, so should never send the application/x-tgsticker mime.
Also, video/lottie+json was recently registered with IANA, so use that instead
of the old image/lottie+json: <https://www.iana.org/assignments/media-types/video/lottie+json>

pkg/connector/capabilities.go

@@ -116,7 +116,7 @@ var fileCaps = event.FileFeatureMap{
 		MimeTypes: map[string]event.CapabilitySupportLevel{
 			"image/webp": event.CapLevelFullySupported,
 			// TODO
-			//"image/lottie+json": event.CapLevelFullySupported,
+			//"video/lottie+json": event.CapLevelFullySupported,
 			//"video/webm": event.CapLevelFullySupported,
 		},
 	},