ci: ignore openjs-foundation.slack.com for markdown link checks

[MikeMcC399]

Description

Add https://openjs-foundation.slack.com to ignorePatterns in https://github.com/nodejs/docker-node/blob/main/markdown_link_check_config.json

Motivation and Context

Links in CONTRIBUTING > Discussion Areas are now causing 403 Forbidden errors when running check:markdown-links:

ERROR: 2 dead links found!
[✖] https://openjs-foundation.slack.com/archives/C0ALS3UDE8G → Status: 403
[✖] https://openjs-foundation.slack.com/archives/C019MGJQ8RH → Status: 403

These links can however be accessed in Google Chrome in an incognito window, so it appears that Slack has increased security against automated checks.

Testing Details

Types of changes

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (none of the above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.