Simplify CI-workflow to run on Ubuntu only

[colinleach]

Two relatively small changes to a GHA workflow:

• Test only on Ubuntu, as proposed in Issue GHA workflows - are we doing too much CI testing? #787.
• In response to an advice message that popped up while I was testing this on GH, change Julia "1.6" to "lts". Those are currently the same, but "lts" should automatically update to 1.10 in the next few months.

I should warn that I have no previous experience of GHA, so getting someone from @exercism/cross-track-maintainers to cast a skeptical eye over it would be good. I see that @ErikSchierboom is on the reviewers list, and it won't let me remove him - apologies for any disturbance!

I ran this manually on my fork, and everything passed (admittedly, not at the first attempt).

[BNAndras]

From experience, Exercism requires (or at least strongly recommends) pinning to specific versions whenever possible. That helps with reproducibility between runs. We can manually upgrade to 1.10 when it’s available. For a point release, it shouldn’t be a big deal hopefully.

[ErikSchierboom]

I wonder if we should go the other way around: testing on three OS-es but only with Julia 1.6?

[depial]

I wonder if we should go the other way around: testing on three OS-es but only with Julia 1.6?

I believe one of the main things that elicited this issue was that Windows + Julia 1.6 takes so long to complete (10 minutes give or take 5) which seems excessive. Would it be a good idea to drop that combo?

[colinleach]

testing on three OS-es but only with Julia 1.6?

We're caught between two separate problems here:

• The 1.6/macos combination caused problems (Upgrade julia version #733, an issue with ARM support), so is currently specifically excluded from the workflow. Erik was involved in that decision, back in May.
• The 1.6/Windows combination is spectacularly slow to run CI tests (later Julia versions are still slower on Windows than on Ubuntu, but not nearly as bad).

One tempting option is to close this PR for now, and revisit the question when 1.6 reaches end-of-life. That is expected some time around November, depending on the release of 1.11 (which is currently at rc4). Then 1.10 becomes the LTS version to test.

[depial]

One tempting option is to close this PR for now, and revisit the question when 1.6 reaches end-of-life. That is expected some time around November, depending on the release of 1.11 (which is currently at rc4). Then 1.10 becomes the LTS version to test.

I think this is a good plan since it's not too far off anyway