🐛Source MailGun: Update start_date validation

[btkcodedev]

What

Closes #27385
Updated start_date validation

How

Updated the pattern

Recommended reading order

1. spec.json

🚨 User Impact 🚨

No breaking changes

[github-actions]

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[btkcodedev]

@marcosmarxm @archangelic Could you prioritize this? Cause, I'm working to migrate it to low code right now
Its just two line change :)

[btkcodedev]

Cause' its raising backward compatibility issues

[marcosmarxm]

Closing this in favor of #29122 which is migrating the connector to low-code.