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>

docs/source/contributing/docs.rst

@@ -13,7 +13,7 @@ Building documentation locally
 We use `sphinx <http://sphinx-doc.org>`_ to build our documentation. It takes
 our documentation source files (written in `markdown
 <https://daringfireball.net/projects/markdown/>`_ or `reStructuredText
-<http://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html>`_ &
+<https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html>`_ &
 stored under the ``docs/source`` directory) and converts it into various
 formats for people to read. To make sure the documentation you write or
 change renders correctly, it is good practice to test it locally.
@@ -39,8 +39,8 @@ change renders correctly, it is good practice to test it locally.
    along with the filename / line number in which they occurred. Fix them,
    and re-run the ``make html`` command to re-render the documentation.
 
-#. View the rendered documentation by opening ``build/html/index.html`` in 
-   a web browser. 
+#. View the rendered documentation by opening ``build/html/index.html`` in
+   a web browser.
 
    .. tip::