use focal instead of jammy distribution

[qasim-at-tci]

Jammy for now is only available for amd64 on the Travis CI platform.

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Name	Status	Preview	Comments	Updated
seqan3	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Jan 18, 2023 at 0:24AM (UTC)

[codecov[bot]]

Codecov Report

Base: 98.18% // Head: 98.18% // No change to project coverage :thumbsup:

Coverage data is based on head (a356bb9) compared to base (613c5f6). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3122 +/- ## ======================================= Coverage 98.18% 98.18% ======================================= Files 276 276 Lines 12292 12292 ======================================= Hits 12069 12069 Misses 223 223 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=seqan). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=seqan)

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[eseiler]

Superseded by #3135, because I can't push to this PR. It's a bit of a dependency-hassle, because focal doesn't really have the compilers we need, except gcc-10.