* update all .github/workflows/ to use the latest version of
actions and the go versions accordingly.
(Only some github actions use a floating tag for the major version.)
* reduce places where the go versions are hardcoded:
* refEr to docs/Development.md from README.md
* use `go.mod` from itest.yml.
* Add an OpenPGP test keypair.
* Move script parts of documentation into script, so they can be used on a fresh Ubuntu 20.04 system
for within a github action to setup a csaf_provider and upload documents to it for an integration test.
* Use dineshsonachalam/markdown-autodocs in github action to automatically
insert lines from the scripts into the docs.
Co-authored-by: Bernhard Reiter <bernhard@intevation.de>