Adds the possibility to also use downloadonly in kwargs

[brejoc]

What does this PR do?

The download_only parameter in the apt module is not in line with the yum and zypper modules. Both of them use downloadonly without the underline.

With this change apt now additionally supports the downloadonly parameter.

What issues does this PR fix or reference?

Fixes #54790

Previous Behavior

The apt module had to be used with download_only.

New Behavior

The apt module can now also be used with downloadonly.

Tests written?

Yes

Commits signed with GPG?

Yes

[brejoc]

I'll add tests once I know that we'd like to take this route.
Tests are added.

[cmcmarrow]

Thanks you for the test and keeping backwards compatibility in mind.

[dwoz]

@brejoc The unit.modules.test_aptpkg.AptPkgTestCase.test_upgrade_downloadonly test is failing.

[brejoc]

This is really strange. Locally the tests are passing and I can see the download-only parameter in the args list. But when run here, the args are just empty:

=============================================================

()
{}
=============================================================

()
{}

[brejoc]

@dwoz looks like this is good to go now. Would it make sense to rebase to master?

[brejoc]

@saltstack/core-pr-reviewers Would it make more sense to close this and open an other PR with these changes against master?

[brejoc]

@dwoz Since this was closed, I opened the same for master: #55448