r/chatbot: Fix SNS topic ARNs ordering in Slack channel configuration

[max-ts0gt]

Description

This PR fixes an inconsistency in the aws_chatbot_slack_channel_configuration resource where it unnecessarily triggers plan changes when SNS topics are returned in a different order from the AWS API.

The fix implements sorting of SNS topic ARNs before comparison in the slackChannelConfigurationHasChanges function, ensuring that the order of topics doesn't trigger unnecessary updates.

Before this fix: Plan would show changes when AWS returns topics in different order:

sns_topic_arns = [
    - "arn:aws:sns:us-west-2:123456789012:topic2"
    - "arn:aws:sns:us-west-2:123456789012:topic1"
    - "arn:aws:sns:us-west-2:123456789012:topic1"
    - "arn:aws:sns:us-west-2:123456789012:topic2"
]

After this fix: No changes shown when only order differs:

sns_topic_arns = [
    - "arn:aws:sns:us-west-2:123456789012:topic1"
    - "arn:aws:sns:us-west-2:123456789012:topic2"
]

Closes #39025 Closes #39870

[github-actions[bot]]

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[johnsonaj]

@max-ts0gt thank you for the contribution! 🎉

I changed sns_topic_arns from a List to Set, which effectively ignores the order without needing to sort.