Fix compatibility issue.

[RenxuanW]

New added field should be the last in serializer.

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[jzhou77]

This is probably not needed, since the main is one major version ahead, thus reordering is fine. I'll leave it to you to decide.

[fdb-windows-ci]

Doxense CI Report for Windows 10

• Commit ID: 05e9ff0
• Result: ✔️ SUCCEEDED
• Build Logs (available for 30 days)

[foundationdb-ci]

AWS CodeBuild CI Report for Linux CentOS 7

• CodeBuild project: foundationdb-pr
• Commit ID: 05e9ff0
• Result: SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)