Update .lycheeignore to include Slack URL

[commjoen]

Added Slack URL to ignore list due to 403 errors.

What kind of changes does this PR include?

• Fixes or refactors
• A new challenge
• Additional documentation
• Something else: fix url checker as slack always gives 403, but channels are correct

Description

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References

Checklist:

• All the contributions made are solely the work of me and my co-authors
• I used AI to generate parts of the content.
• I tested the changes in this PR (if applicable)
• I added unit tests to ensure my change works (when change in Java or on front-end code)
• I added UI tests to ensure my UI changes work (when change in the overall UI, not needed if just adding a challenge)
• The PR passes pre-commit hooks and automated tests