update to go1.21.12

- https://github.com/golang/go/issues?q=milestone%3AGo1.21.12+label%3ACherryPickApproved
- full diff: https://github.com/golang/go/compare/go1.21.11...go1.21.12

These minor releases include 1 security fixes following the security policy:

net/http: denial of service due to improper 100-continue handling

The net/http HTTP/1.1 client mishandled the case where a server responds to a request with an "Expect: 100-continue" header with a non-informational (200 or higher) status. This mishandling could leave a client connection in an invalid state, where the next request sent on the connection will fail.

An attacker sending a request to a net/http/httputil.ReverseProxy proxy can exploit this mishandling to cause a denial of service by sending "Expect: 100-continue" requests which elicit a non-informational response from the backend. Each such request leaves the proxy with an invalid connection, and causes one subsequent request using that connection to fail.

Thanks to Geoff Franks for reporting this issue.

This is CVE-2024-24791 and Go issue https://go.dev/issue/67555.

View the release notes for more information:
https://go.dev/doc/devel/release#go1.21.12

**- Description for the changelog**

```markdown changelog
Update Go runtime to 1.21.12
```

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

Dockerfile

@@ -1,6 +1,6 @@
 # syntax=docker/dockerfile:1.7
 
-ARG GO_VERSION=1.21.11
+ARG GO_VERSION=1.21.12
 ARG BASE_DEBIAN_DISTRO="bookworm"
 ARG GOLANG_IMAGE="golang:${GO_VERSION}-${BASE_DEBIAN_DISTRO}"
 ARG XX_VERSION=1.4.0