Task: automate updating the certificates bundle

[jrfnl]

Thoughts:

• Use GH Actions with a daily cron schedule
• Check for changes to this page and/or RSS feed if available: https://curl.se/docs/caextract.html
• Use similar logic as used in the website update workflow to auto-generate a PR when a new bundle has been published.

[wojsmol]

We can simply wget https://curl.se/ca/cacert.pem and create PR if changes are detected.

[jrfnl]

FYI: I have this ready and waiting for a new certificate bundle to be published, so I can actually test that the workflow works as expected ;-)

[jrfnl]

Just saw that a new certificates bundle has been published on https://curl.se/docs/caextract.html, so I can now (finally) pull the new workflow. If all goes well, a separate PR will automatically be opened with the certificate update.
Actually, for a PR which updates the workflow or the bundles, a new commit should be added to the open PR (if all goes well).

See: #669