Update to go version 1.21.1

[BramVanBerkel]

In my exercises i came across a few instances where I was using go 1.21.1 locally, but the testrunner was running 1.20. This resulted in tests that passed locally, but not in the testrunner.

1.21 includes some new functions, most notably for me new functions for slices

We can also update the alpine image from 3.17 to 3.18 but was unsure if i also should include that in this PR, maybe that is better off in a new one.

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[junedev]

I am preliminary re-opening the PR to see what CI has to say about this. Not sure whether there were changes to the test outputs again which would mean this is more work than changing a number.

[junedev]

@BramVanBerkel I noticed that if the docker container uses 1.21, we also need to use that for our tests. I pushed that change to your PR but as expected some tests are failing now, probably because the test output changed slightly. Can you have a look at fixing the tests so this can be merged?

[BramVanBerkel]

I've fixed the tests, can the workflows be ran? 😄