Reduce generated project jobs to steps

[santacodes]

The previous CI job used 64 jobs to test all the combinations of generated projects using copier; now the number of combinations has reduced to 16. I think it would be better to keep python versions into their separate matrix as it would be much more cleaner and readable that way, personally I don't think it would be worth it to split them into their own steps with multiple python virtual environments, writing hacky scripts to generate and run tests on generated projects.

Fixes #117

[agriyakhetarpal]

Thanks, @santacodes! I see that there are two annotations for the CI run at https://github.com/pybamm-team/pybamm-cookie/actions/runs/15229557825?pr=119, could you also fix them? Perhaps it's just an extraneous input?

Also, I think we can reduce this even further. On the same link, you can see that we have 16 jobs (and therefore 16 runners) for the template_test matrix, and then we have tests for all of them in 16 jobs in generated_project_tests. Your idea of keeping Python versions in separate jobs is alright – I agree there. However, running the generated project tests as a step within template_test should reduce 16 jobs from the current 36.