Deprecate the legacy option for --summary-mode

[joanlopez]

In #4089, we modified the end-of-test summary appearance, replacing the existing summary with a revamped one.

As that also introduced breaking changes for those users with a custom handleSummary and/or using the --summary-export method flag, we decided to keep an option named legacy, for an opt-in backward compatibility.

However, near the v2 release, at some point, with enough time in advance, we should add a warning, to tell users still relying on the legacy summary, that we'll drop that support completely.

1. Mark legacy summary mode as deprecated (display a warning).
2. (Likely on v2.0, create another issue once ready) Remove support for legacy summary mode.