[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 5c1567836c)
(cherry picked from commit 953afbc67f)
(cherry picked from commit fd9d97ab9f)
(cherry picked from commit 40fbd45eff)
(cherry picked from commit c5c904b04b)
(cherry picked from commit 48659bb3ab)
(cherry picked from commit b1fccd5093)
(cherry picked from commit 5e69573860)

modules/updatechecker/update_checker_test.go

@@ -0,0 +1,16 @@
+// Copyright 2023 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package updatechecker
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDNSUpdate(t *testing.T) {
+	version, err := getVersionDNS("release.forgejo.org")
+	assert.NoError(t, err)
+	assert.NotEmpty(t, version)
+}