[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 0baefb546a)
(cherry picked from commit e8ee41880b)
(cherry picked from commit 7eca4f3bf1)
(cherry picked from commit 6dde3992dc)
(cherry picked from commit fb3a37fbfc)
(cherry picked from commit 8304af1e9d)
(cherry picked from commit 0543a7d12a)
(cherry picked from commit c3a22933b7)
(cherry picked from commit e243707694)
(cherry picked from commit 7eb6d1bcf7)
(cherry picked from commit 1d7b9535cd)

services/cron/tasks_extended.go

@@ -142,7 +142,8 @@ func registerDeleteOldActions() {
 func registerUpdateGiteaChecker() {
 	type UpdateCheckerConfig struct {
 		BaseConfig
-		HTTPEndpoint string
+		HTTPEndpoint   string
+		DomainEndpoint string
 	}
 	RegisterTaskFatal("update_checker", &UpdateCheckerConfig{
 		BaseConfig: BaseConfig{
@@ -150,10 +151,11 @@ 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)
 	})
 }