[PRIVACY] Add a DNS method to fetch new updates

- Use TXT records in order to determine the latest available version.
- This addresses a valid privacy issue, as with HTTP requests the server
can keep track(estimated) of how many instances are using Forgejo, with
DNS that's basically not possible as the server will never receive any
data, as the only ones receiving data are DNS resolvers.

(cherry picked from commit e68aaa0c16)

services/cron/tasks_extended.go

@@ -147,7 +147,8 @@ func registerDeleteOldActions() {
 func registerUpdateGiteaChecker() {
 	type UpdateCheckerConfig struct {
 		BaseConfig
-		HTTPEndpoint string
+		HTTPEndpoint   string
+		DomainEndpoint string
 	}
 	RegisterTaskFatal("update_checker", &UpdateCheckerConfig{
 		BaseConfig: BaseConfig{
@@ -155,10 +156,11 @@ func registerUpdateGiteaChecker() {
 			RunAtStart: false,
 			Schedule:   "@every 168h",
 		},
-		HTTPEndpoint: "https://dl.gitea.io/gitea/version.json",
+		HTTPEndpoint:   "https://dl.gitea.io/gitea/version.json",
+		DomainEndpoint: "release.forgejo.org",
 	}, func(ctx context.Context, _ *user_model.User, config Config) error {
 		updateCheckerConfig := config.(*UpdateCheckerConfig)
-		return updatechecker.GiteaUpdateChecker(updateCheckerConfig.HTTPEndpoint)
+		return updatechecker.GiteaUpdateChecker(updateCheckerConfig.HTTPEndpoint, updateCheckerConfig.DomainEndpoint)
 	})
 }