gpg api: allow self signed and use default gpg version

[neolynx]

Description of the Change

Self signed GPG keys were refused, which is something aptly should allow, if a repo for mirroring is singed this way.

Also, use the default GPG version, as in the other parts for using GPG keys.

The API also returns the gpg command output, instead of just a failure.

Checklist

• [ ] unit-test added (if change is algorithm)
• [ ] functional test added/updated (if change is functional)
• [ ] man page updated (if applicable)
• [ ] bash completion updated (if applicable)
• [ ] documentation updated

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 74.78%. Comparing base (e564b7c) to head (eda1e48).

Files	Patch %	Lines
api/gpg.go	60.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1246 +/- ## ======================================= Coverage 74.78% 74.78% ======================================= Files 143 143 Lines 16200 16200 ======================================= Hits 12115 12115 Misses 3148 3148 Partials 937 937 ```

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