Use non-version specific reference URLs for tool version workflow variables

GitHub Actions actions are used by the workflows to set up development tools in the runner workspace.

In order to facilitate updates to new versions of these tools, we set the version to be set up via environment variables
at the top of the workflow.

Since this variable definition is separate from the step using the action, it might not be immediately apparent to the
maintainer which version syntaxes are supported. For this reason, comments were added with the URL to the relevant
section of the consuming action's documentation. Previously, these URLs were made to point to the version of the
documentation that matched the version of the action in use by the workflow. Since we only use a major version ref, the
expectation was that this would only need to be updated rarely. However, it turned out that the major version bump cycle
is significantly shorter than expected. In addition, it is easy to forget the update because action version update PRs
are provided by Dependabot, which obviously won't update the URLs in the comments.

So it will be best to use a URL that points to the documentation at the tip of the default branch of the action
repository. The likelihood of the documentation provided by this URL not matching the behavior of the release version of
the action in use is likely less than it is for an outdated URL.

Author: per1234

.github/workflows/release.yml

@@ -7,7 +7,7 @@ env:
   ARTIFACT_NAME: dist
   # The project's folder on Arduino's download server for uploading builds
   AWS_PLUGIN_TARGET: /tools/
-  # See: https://github.com/actions/setup-python/tree/v3#available-versions-of-python
+  # See: https://github.com/actions/setup-python/tree/main#available-versions-of-python
   PYTHON_VERSION: "3.7"
   MCUBOOT_PATH: ${{ github.workspace }}/mcuboot
   IMGTOOL_PACKING_PATH: ${{ github.workspace }}/imgtool-packing