Allow specifying SECRET_KEY_URI, similar to INTERNAL_TOKEN_URI (#19663)

Only load SECRET_KEY and INTERNAL_TOKEN if they exist.
Never write the config file if the keys do not exist, which was only a fallback for Gitea upgraded from < 1.5

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/setting/lfs.go

@@ -62,7 +62,7 @@ func newLFSService() {
 			}
 
 			// Save secret
-			CreateOrAppendToCustomConf(func(cfg *ini.File) {
+			CreateOrAppendToCustomConf("server.LFS_JWT_SECRET", func(cfg *ini.File) {
 				cfg.Section("server").Key("LFS_JWT_SECRET").SetValue(LFS.JWTSecretBase64)
 			})
 		}