package/go: bump to version 1.21.0

Go 1.21.0 is a major release of Go.

https://go.dev/doc/devel/release#go1.21.0

Set GOTOOLCHAIN=local to disable the new toolchain download feature. This
feature, introduced in Go 1.21.x, will automatically download pre-built compiler
binaries from Google for the toolchain version specified in go.mod. We do not
want this in Buildroot as we build from source instead: set GOTOOLCHAIN=local to
disable the feature and use the locally built toolchain.

https://go.dev/doc/toolchain

Signed-off-by: Christian Stewart <christian@aperture.us>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/go/go.hash

@@ -1,3 +1,3 @@
 # From https://go.dev/dl
-sha256  2c5ee9c9ec1e733b0dbbc2bdfed3f62306e51d8172bf38f4f4e542b27520f597  go1.20.7.src.tar.gz
+sha256  818d46ede85682dd551ad378ef37a4d247006f12ec59b5b755601d2ce114369a  go1.21.0.src.tar.gz
 sha256  2d36597f7117c38b006835ae7f537487207d8ec407aa9d9980794b2030cbc067  LICENSE

package/go/go.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-GO_VERSION = 1.20.7
+GO_VERSION = 1.21.0
 GO_SITE = https://storage.googleapis.com/golang
 GO_SOURCE = go$(GO_VERSION).src.tar.gz
 
@@ -28,6 +28,7 @@ HOST_GO_COMMON_ENV = \
 	GOCACHE="$(HOST_GO_TARGET_CACHE)" \
 	GOMODCACHE="$(HOST_GO_GOPATH)/pkg/mod" \
 	GOPROXY=off \
+	GOTOOLCHAIN=local \
 	PATH=$(BR_PATH) \
 	GOBIN= \
 	CGO_ENABLED=$(HOST_GO_CGO_ENABLED)