Deprecated support for legacy tasks in V2 workflows

[zonia3000]

Checklist before merging

• [X] I added an appropriate entry to CHANGELOG.md

Closes #545

[tcompa]

Thanks!

Small remark: Task.is_v2_compatible (in https://github.com/fractal-analytics-platform/fractal-web/blob/e929c7e1f6e4f68ef220280690a173efa2329916/src/lib/types.d.ts) is not present any more.

[tcompa]

Two other files to check (if relevant):

• sandbox/src/lib/components/VersionUpdateSandbox.svelte has some is_legacy_task: false
• src/lib/components/v1/workflow/version-checker.js has some const response = await fetch(v2 ? `/api/v2/task-legacy` : `/api/v1/task`);