tpl/tplimpl: Deprecate gist shortcode

Closes #13211

tpl/tplimpl/embedded/templates/shortcodes/gist.html

@@ -1 +1,2 @@
+{{- warnf "The %q shortcode was deprecated in v0.143.0 and will be removed in a future release. See https://gohugo.io/shortcodes/gist for instructions to create a replacement." .Name -}}
 <script src="https://gist.github.com/{{ index .Params 0 }}/{{ index .Params 1 }}.js{{if len .Params | eq 3 }}?file={{ index .Params 2 }}{{end}}"></script>

tpl/tplimpl/shortcodes_integration_test.go

@@ -30,6 +30,9 @@ disableKinds = ['page','rss','section','sitemap','taxonomy','term']
 -- layouts/index.html --
 {{ .Content }}
 -- content/_index.md --
+---
+title: home
+---
 a{{< comment >}}b{{< /comment >}}c
 `
 
@@ -87,6 +90,26 @@ E: An _emphasized_ word.
 	)
 }
 
+func TestGistShortcode(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['page','rss','section','sitemap','taxonomy','term']
+-- layouts/index.html --
+{{ .Content }}
+-- content/_index.md --
+---
+title: home
+---
+{{< gist jmooring 23932424365401ffa5e9d9810102a477 >}}
+`
+
+	b := hugolib.Test(t, files, hugolib.TestOptWarn())
+	b.AssertFileContent("public/index.html", `<script src="https://gist.github.com/jmooring/23932424365401ffa5e9d9810102a477.js"></script>`)
+	b.AssertLogContains(`WARN  The "gist" shortcode was deprecated in v0.143.0 and will be removed in a future release. See https://gohugo.io/shortcodes/gist for instructions to create a replacement.`)
+}
+
 func TestInstagramShortcode(t *testing.T) {
 	t.Parallel()