Backend: Remove str-based EventUpdate items

[tristanlabelle]

Removes the deprecated updated_str_items field from both the EventUpdate notification data message and the GenerateEventUpdateNotificationsPayload internal job payload, reserving the field numbers and names. The i18n-friendly updated_enum_items field is now the sole source.

(a month has passed since I migrated us over so it should be safe that there are no old queued jobs using updated_str_items anymore.)

Fixes #9117

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📝 Release Notes

This PR does not need to be included in release notes.

Reason: This PR removes deprecated backend/internal notification fields and cleanup code after a migration. It does not add or change any noticeable user-facing behavior, and the technical work is routine maintenance rather than a major infrastructure or reliability improvement worth highlighting to end users.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This PR removes deprecated backend/internal notification fields and cleanup code after a migration. It does not add or change any noticeable user-facing behavior, and the technical work is routine maintenance rather than a major infrastructure or reliability improvement worth highlighting to end users.