clarified Mercurial version: presumably the last version that supports both python2 and python3;

--- a/src/Pure/Tools/phabricator.scala	Tue Jan 23 21:00:54 2024 +0100
+++ b/src/Pure/Tools/phabricator.scala	Tue Jan 23 23:15:51 2024 +0100
@@ -205,7 +205,7 @@
   def standard_mercurial_source: String = {
     val release = Linux.Release()
     if (release.is_ubuntu_20_04) "https://www.mercurial-scm.org/release/mercurial-3.9.2.tar.gz"
-    else "https://www.mercurial-scm.org/release/mercurial-6.1.1.tar.gz"
+    else "https://www.mercurial-scm.org/release/mercurial-6.1.4.tar.gz"
   }