Write all logging (INFO, WARN, ERROR) to stderr

The old setup tried to log >= warning to stderr, the rest to stdout.

However, that logic was flawed, so warnings ended up in stdout, which makes `hugo list all` etc. hard to reason about from scripts.

This commit fixes this by making all logging (info, warn, error) log to stderr and let stdout be reserved for program output.

Fixes #13074

testscripts/commands/warnf_stderr.txt

@@ -0,0 +1,13 @@
+# Issue #13074
+
+hugo
+stderr 'warning'
+! stdout 'warning'
+
+-- hugo.toml --
+baseURL = "http://example.org/"
+disableKinds = ["RSS", "page", "sitemap", "robotsTXT", "404", "taxonomy", "term"]
+-- layouts/index.html --
+Home
+{{ warnf "This is a warning" }}
+