Drop "unrolled/render" package (#23965)

None of the features of `unrolled/render` package is used. 

The Golang builtin "html/template" just works well. Then we can improve
our HTML render to resolve the "$.root.locale.Tr" problem as much as
possible.

Next step: we can have a template render pool (by Clone), then we can
inject global functions with dynamic context to every `Execute` calls.
Then we can use `{{Locale.Tr ....}}` directly in all templates , no need
to pass the `$.root.locale` again and again.

tests/integration/markup_external_test.go

@@ -27,7 +27,7 @@ func TestExternalMarkupRenderer(t *testing.T) {
 	const repoURL = "user30/renderer"
 	req := NewRequest(t, "GET", repoURL+"/src/branch/master/README.html")
 	resp := MakeRequest(t, req, http.StatusOK)
-	assert.EqualValues(t, "text/html; charset=UTF-8", resp.Header()["Content-Type"][0])
+	assert.EqualValues(t, "text/html; charset=utf-8", resp.Header()["Content-Type"][0])
 
 	bs, err := io.ReadAll(resp.Body)
 	assert.NoError(t, err)