[Codacy] Change this constant to a 'static' read-only property.

[github-actions]

Codacy finding

• Repository: TransactionProcessing/SupportTools
• Codacy org: TransactionProcessing
• Codacy repo: SupportTools
• Codacy issue ID: 18e4bbfdda297f7c5171c95a64812295
• Codacy issue: https://app.codacy.com/gh/TransactionProcessing/SupportTools/issues/18e4bbfdda297f7c5171c95a64812295
• File: TransactionProcessing.MerchantFileProcessor/Configuration/TransactionFileFieldSources.cs
• Line: 9
• Rule: SonarCSharp_S2339
• Severity: Warning

Message: Change this constant to a 'static' read-only property.

Sync metadata

Sync key: TransactionProcessing/SupportTools::TransactionProcessing/SupportTools::18e4bbfdda297f7c5171c95a64812295