dockerfile: graduate ADD --checksum=<checksum> from labs

e.g.,
```
ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d \
  https://mirrors.edge.kernel.org/pub/linux/kernel/Historic/linux-0.01.tar.gz /
```

Introduced to dockerfile:1.5-labs in PR 3093

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Author: AkihiroSuda

.golangci.yml

@@ -7,7 +7,6 @@ run:
 
   build-tags:
     - dfrunsecurity
-    - dfaddchecksum
 
 linters:
   enable:

frontend/dockerfile/dockerfile2llb/convert.go

@@ -1055,9 +1055,6 @@ func dispatchCopy(d *dispatchState, cfg copyConfig) error {
 		if !cfg.isAddCommand {
 			return errors.New("checksum can't be specified for COPY")
 		}
-		if !addChecksumEnabled {
-			return errors.New("instruction 'ADD --checksum=<CHECKSUM>' requires the labs channel")
-		}
 		if len(cfg.params.SourcePaths) != 1 {
 			return errors.New("checksum can't be specified for multiple sources")
 		}

frontend/dockerfile/dockerfile2llb/convert_addchecksum.go

@@ -1,6 +1,3 @@
-//go:build dfaddchecksum
-// +build dfaddchecksum
-
 package dockerfile
 
 import (

frontend/dockerfile/dockerfile2llb/convert_noaddchecksum.go

@@ -1298,6 +1298,7 @@ guide – Leverage build cache](https://docs.docker.com/develop/develop-images/d
 > **Note**
 >
 > Not yet available in stable syntax, use [`docker/dockerfile:1-labs`](#syntax) version (`1.5-labs` or newer).
+> Planned to be included in `docker/dockerfile:1.6`.
 
 The checksum of a remote file can be verified with the `--checksum` flag:
 

frontend/dockerfile/dockerfile_addchecksum_test.go

@@ -1 +1 @@
-dfrunsecurity dfaddchecksum
+dfrunsecurity