[16.0][IMP] fieldservice_calendar: Sync description

[houzefa-abba]

This PR contains 2 commits: 1 to modernize code related to this field sync; 1 which adds description sync.

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Propagate description updates between FSM order & calendar event.

1 is an HTML field, the other is not; we handle conversion.

Before this change, description was only propagated at calendar event creation, and without text➔html conversion.

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To test:

• Field Service > Configuration > Settings: Enable "Manage Teams"
• Field Service > Configuration > Teams: Set a "Team's calendar"
• Create an FSM order, give it a scheduled date & duration
• Update FSM order description, observe updates in calendar event

FSM order with multiline description & schedule:

Calendar event with that multiline description (thanks to text➔html conversion):