discord: proper support for discriminator-less

names with no discriminator are shown as @username, and search works
with the @ too. Also bumped go version to 1.20, since it was stuck on
1.16 and i felt like trying generics (in an ugly way).

api-users.go

@@ -916,7 +916,8 @@ func (app *appContext) GetUsers(gc *gin.Context) {
 			user.NotifyThroughMatrix = mxUser.Contact
 		}
 		if dcUser, ok := app.storage.discord[jfUser.ID]; ok {
-			user.Discord = dcUser.Username + "#" + dcUser.Discriminator
+			user.Discord = RenderDiscordUsername(dcUser)
+			// user.Discord = dcUser.Username + "#" + dcUser.Discriminator
 			user.DiscordID = dcUser.ID
 			user.NotifyThroughDiscord = dcUser.Contact
 		}