Missing fixes, docs and deprecations rework

[lkubb]

Some minor fixes/improvements that were not part of the PR and further cleanup.

Additionally:

• Remove versionadded/versionchanged from previous Salt releases (irrelevant for a new project)
• Change versionadded/versionchanged from 3007.0 to 1.0.0 to highlight the differences vs Salt core
• Change calls to salt.utils.versions.warn_until to a local analog since we're independent from Salt releases now.
• Ensure we have a path to migrate from some currently broken (but backwards-compatible) defaults to more sensible ones (SDB patch behavior false -> true and vault.list_secrets returning the list instead of a single-key dict {keys: [secret/keys, we/actually/asked/for]}

After this, I think the following still needs to happen before a release:

• I remember there was an issue with timestamps and time zones. Looking into this now. Nothing to fix, it's fine.
• Ensure we have a working changelog setup, open equivalent issues here and add them to the changelog.
• At least make the README a bit more user-friendly. I would love to rework the docs since we're not limited to docstrings anymore, but that shouldn't be a blocker and would likely heavily benefit from user feedback/involvement. I'm far too deep in the workings of this code. :)
• (Publish the docs at readthedocs.)
• (Also add issue/PR templates)