fix: use correct ACME default (#8550)

- The `ACME_URL` setting is documented to default to Let's encrypt
production server if left empty, so do precisely that.
- Use a HTTP proxy to communicate with ACME if Forgejo is configured to
use that.
- Regression of forgejo/forgejo#7409 (previously certmagic took care of
setting these defaults).
- Resolves forgejo/forgejo#8548

## Testing

1. Configure Forgejo's root URL to a public facing domain (that can pass a ACME challenge)
2. Configure Forgejo to use ACME by setting `[server].ENABLE_ACME = true` and `[server].ACME_ACCEPTTOS = true`.
3. Start Forgejo.
4. Observe that it's available via https.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8550
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

modules/setting/server.go

@@ -16,6 +16,8 @@ import (
 	"forgejo.org/modules/json"
 	"forgejo.org/modules/log"
 	"forgejo.org/modules/util"
+
+	"github.com/caddyserver/certmagic"
 )
 
 // Scheme describes protocol types
@@ -206,7 +208,7 @@ func loadServerFrom(rootCfg ConfigProvider) {
 			EnableAcme = sec.Key("ENABLE_LETSENCRYPT").MustBool(false)
 		}
 		if EnableAcme {
-			AcmeURL = sec.Key("ACME_URL").MustString("")
+			AcmeURL = sec.Key("ACME_URL").MustString(certmagic.LetsEncryptProductionCA)
 			AcmeCARoot = sec.Key("ACME_CA_ROOT").MustString("")
 
 			if sec.HasKey("ACME_ACCEPTTOS") {