Fix regexp of slack webhooks in gitleaks file

What does this MR do?

While running the secrets analyzer on a project we saw that slack webhooks aren't detected by the analyzer. A fix for the regexp was introduced in the upstream repo here so I adapted the gitleaks file to include the fix.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer