Rename the system tests GitHub Actions workflows

[MakisH]

This PR renames/cleans up the CI workflows of the system tests to make them more consistent and intuitive.

Overview:

• run_testsuite_workflow.yml -> system-tests.yml
• run_testsuite_manual.yml is removed, as it is now essentially the same as system-tests-latest-components.yml. I keep the name to reflect that the system tests use the latest components by default, unless the user provides inputs to override them.
• generate_reference_results_manual.yml -> generate-reference-results-manual.yml, for consistency
• generate_reference_results_workflow.yml -> generate-reference-results.yml

The names of the workflows are also adjusted:

• System tests (latest components) -> System tests (manual/nightly)
• Run Testsuite reusable workflow -> System tests (called)
• Generate reference results (manual) remains
• Generate reference results workflow -> Generate reference results (called)

A side-effect of this will be that the previous names will still be visible in the sidebar, similarly to other older workflows. We work around this by pinning the relevant workflows.

Checklist

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.