[CI] Address Hugo deprecation warning: use hugo.IsMultilingual

[chalin]

• Fixes [infra] Hugo reports deprecation warning for .Site.IsMultiLingual #4957
• There are no changes to the generated site files, other than the site timestamp:

  $ (cd public && git diff -bw) | grep ^diff
  diff --git a/site/index.html b/site/index.html

/cc @tiffany76 @cartermp