[Playground] [Backend] Removing unused snippets manually and using the scheduled task

[vchunikhin]

fixes #22296

Removing unused code snippets in the Google Cloud Datastore using the last visit date for filtering

@VladMatyunin

This PR depends on next PRs:

• [Bug fix] #22310

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[vchunikhin]

R: @pabloem

[github-actions[bot]]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[pabloem]

lgtm