SLA Calculations 2/2: Simplify logic

[valentijnscholten]

If #11923 gets merged, we have unit tests to capture the SLA calculation behaviour.

So we can now simplify the logic to make it easier to understand, maintain and (possibly) add business days support.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

Conflicts have been resolved. A maintainer will review the pull request shortly.

[dryrunsecurity]

DryRun Security Summary

The PR enhances SLA-related functionality in the Finding model by updating expiration date calculation, modifying template tag variables, and adding comprehensive unit tests, while also addressing potential security risks in test logging and data handling.

Expand for full summary

The PR updates the SLA expiration date calculation in the Finding model, modifies template tag variable names, and adds comprehensive SLA calculation unit tests.

Security findings:

1. Potential information disclosure risk in test logging (unittests/test_sla_calculations.py): Logging statements may print finding and SLA configuration details that could expose sensitive information if logs are not properly secured.
2. Test data handling risk (unittests/test_sla_calculations.py): Uses a fixture and hardcoded test data that might contain sensitive credentials or test information.

View PR in the DryRun Dashboard.