chore(local): clear ambiguity in between sg version|live (#64122)

Context: As I was catching up with my Slack notifications, I spotted
this
[conversation](https://sourcegraph.slack.com/archives/C04MYFW01NV/p1721137862832899?thread_ts=1721136881.733869&cid=C04MYFW01NV)
and this PR is a 5m fix to avoid the problem to happen again.

What: If you haven't used `sg` in a while, it's easy to think that `sg
version` refers to the currently deployed Sourcegraph instance and not
the CLI. This commit adds a little preamble on stderr to not mess with
script usage while still reminding the user that it's the CLI version
that gets printed out.

Before: we printed the version without any context
After: we also print the following on stderr: `👉 Showing the current
version of the sg CLI, if you're looking for deployed Sourcegraph
instances version, please use `sg live` instead.`. Stderr so we don't
break things like `$(sg version)`.

Note: at a broader level, we should generalize this pattern, we don't
use too much `sg` raw output in scripts, but for the few places where
it's the case, it's a footgun.

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## Test plan

Locally tested + CI. 

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## Changelog

- `sg version` explicitly mentions that it's the CLI version that's
printed out, not any instance version.

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dev/sg/sg_version.go

@@ -49,6 +49,10 @@ var (
 )
 
 func versionExec(c *cli.Context) error {
+	// Write on stderr to ensure we can use the output in scripts without having to trim
+	// the output from the contextual infos.
+	outErr := std.NewOutput(os.Stderr, verbose)
+	outErr.WriteNoticef("Showing the current version of the sg CLI, if you're looking for deployed Sourcegraph instances version, please use `sg live` instead.")
 	if verbose {
 		std.Out.Writef("Version: %s\nBuild commit: %s",
 			c.App.Version, BuildCommit)