Backport PR #3462: prepare to rename default branch to main

- update references to default branch name in docs, workflows
- use HEAD in github urls, which always works regardless of default branch name
- fix petstore URLs since the old petstore links seem to have stopped working

to merge, in order:

- [x] approve this PR
- [x] rename the default branch to main in settings
- [x] merge this PR

Related tangent: I've been using [this git default-branch](https://github.com/minrk/git-stuff/blob/main/bin/git-default-branch) to help with my aliases and friends working with repos with different branch names.

Signed-off-by: Min RK <benjaminrk@gmail.com>

examples/bootstrap-script/bootstrap.sh

@@ -40,9 +40,9 @@ else
     echo "...initial content loading for user ..."
     mkdir $USER_DIRECTORY/tutorials
     cd $USER_DIRECTORY/tutorials
-    wget https://github.com/jakevdp/PythonDataScienceHandbook/archive/master.zip
-    unzip -o master.zip
-    rm master.zip
+    wget https://github.com/jakevdp/PythonDataScienceHandbook/archive/HEAD.zip
+    unzip -o HEAD.zip
+    rm HEAD.zip
 fi
 
 exit 0