[24.0 backport] Dockerfile.vendor: update GOPROXY to use default with fallback

[thaJeztah]

• backport of Dockerfile.vendor: update GOPROXY to use default with fallback #4327
• relates to cli/command: fix GoDoc referencing wrong const #4324 (comment)
• relates to [master] Update GOPROXY to use default with fallback docker-ce-packaging#745

Use the default proxy, to assist with vanity domains mis-behaving, but keep a fallback for situations where we need to get modules from GitHub directly.

This should hopefully help with the gopkg.in/yaml.v2 domain often going AWOL;

#14 245.9 	gopkg.in/yaml.v2@v2.4.0: unrecognized import path "gopkg.in/yaml.v2": reading https://gopkg.in/yaml.v2?go-get=1: 502 Bad Gateway
#14 245.9 	server response: Cannot obtain refs from GitHub: cannot talk to GitHub: Get https://github.com/go-yaml/yaml.git/info/refs?service=git-upload-pack: write tcp 10.131.9.188:60820->140.82.121.3:443: write: broken pipe

curl 'https://gopkg.in/yaml.v2?go-get=1'
Cannot obtain refs from GitHub: cannot talk to GitHub: Get https://github.com/go-yaml/yaml.git/info/refs?service=git-upload-pack: write tcp 10.131.9.188:60820->140.82.121.3:443: write: broken pipe

From the Go documentation; https://go.dev/ref/mod#goproxy-protocol

List elements may be separated by commas (,) or pipes (|), which determine error
fallback behavior. When a URL is followed by a comma, the go command falls back
to later sources only after a 404 (Not Found) or 410 (Gone) response. When a URL
is followed by a pipe, the go command falls back to later sources after any error,
including non-HTTP errors such as timeouts. This error handling behavior lets a
proxy act as a gatekeeper for unknown modules. For example, a proxy could respond
with error 403 (Forbidden) for modules not on an approved list (see Private proxy
serving private modules).

(cherry picked from commit 6458dcb)

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