Fix all go version in the tests on 1.20

[ineiti]

There is a regression error in go 1.22.
This PR fixes the go version to 1.20.x:

• updated go.mod
• updated github workflows
• removed rand.Seed from integration tests (which was useless anyway, because of non-deterministic go-routines)

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[PascalinDe]

LGTM, but why was the seed fixed in the first place?

[ineiti]

They wanted to have deterministic tests. So anything depending on randomness would give the same result on every run.

But if the go-routines are not deterministic (and they aren't), and if the go-routines take data from the random-number generator, the outcome is not deterministic anymore. Or at least most of the time. Unless you use Antithesis :)