[Discover][Search solution] Set @timestamp as the default data view time field

[jughosta]

Summary

This PR sets @timestamp as the main time field for the default data view in Search Solution. This will fix Discover UX in Data View mode:

• histogram would appear
• @timestamp column would be added to the grid
• time range filter would appear next to the search bar
• it would be allowed to create Alerts

Before the changes:

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 53a5a3f

Metrics [docs]

✅ unchanged

History

• 💛 Build #250377 was flaky 3bd2036
• 💔 Build #250338 failed 7111e35

cc @jughosta

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[elasticmachine]

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