Ignore StackExchange.Redis version updates in Dependabot

[jbrinkman]

Problem

Dependabot is currently configured to update all NuGet packages, including StackExchange.Redis. However, this project needs to maintain compatibility with StackExchange.Redis version 2.8.58.

Solution

Update the .github/dependabot.yml configuration to ignore version updates for StackExchange.Redis to prevent automatic updates that could break compatibility.

Acceptance Criteria

• Dependabot ignores StackExchange.Redis version updates
• Other NuGet package updates continue to work normally
• Configuration maintains compatibility with version 2.8.58