[Common] Prevent dangling reference error when switching from gsl::span to std::span

[aalkin]

In the original code a temporary iterator to fv0a/fv0c is created and immediately discarded, leaving the result of amplitude() (which is a span, a non-owning view) a with dangling references to the internals of the iterator. This worked by accident, since the underlying array in shared memory is available regardless, but can potentially lead to problems. Switching from gsl::span to the std::span (PR AliceO2Group/AliceO2#14421) makes the original code fail to compile.

@ddobrigk @ktf

[github-actions]

O2 linter results: ❌ 84 errors, ⚠️ 0 warnings, 🔕 0 disabled

[ddobrigk]

Hi @aalkin, this is an impressively sneaky issue, thanks for the fix!

In practice, note you'll need a lot of approvals for this now (DPG, PWGCF and Common), so I fear you may have to ping people in other channels (e.g. mattermost) to get this merged in good time.

[aalkin]

The issue is solved at the framework side.