Fix the bug that connector starts sync immediately when scheduled

[wangch079]

Closes https://github.com/elastic/enterprise-search-team/issues/3366

The connector will start syncing immediately if its scheduling is enabled and it's never synced.

This PR fixes this bug, and makes the job scheduling respect the crontab:

It gets the next sync time from now, and compares it with now + poll_interval. It will trigger the sync if the next sync time happens before the next poll. This is consistent with the python implementation.

A follow-up PR to update ent-search gem: https://github.com/elastic/ent-search/pull/7322

Checklists

Pre-Review Checklist

• [x] Covered the changes with automated tests
• [x] Tested the changes locally
• [x] Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)
• [ ] Considered corresponding documentation changes
• [ ] Contributed any configuration settings changes to the configuration reference

[github-actions[bot]]

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[wangch079]

💚 All backports created successfully

Status	Branch	Result
✅	8.7

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