Docs: fix Audit Events badge

What does this MR do?

Updates badge from Starter only to Starter/Bronze.

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9360

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index == N/A
• Crosslink the document from other subject-related docs == N/A
• Feature moving tiers? Make sure the change is also reflected in features.yml == N/A
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails == N/A

Review checklist

• Your team's review (required) == required in this case?
• PM's review (recommended, but not a blocker)
• Technical writer's review (required) @eread
• Merge the EE-MR first, CE-MR afterwards