clarify voucher wording

[abbycabs]

@chriswblake

[Copilot]

Pull Request Overview

This PR clarifies the wording around certification vouchers and refines the skills courses list on the Security Challenge page.

• Reorders the “Introduction to Repository Management” course to the top of the list and removes its duplicate
• Updates all voucher references to explicitly mention the “certification exam” and adjusts related deadlines
• Enhances Step 3 heading and description to better reflect the exam process

Comments suppressed due to low confidence (1)

pages/security-challenge.js:54

• [nitpick] The heading for Step 3 uses a period instead of a colon, which is inconsistent with the Step 2 heading. Consider changing ### Step 3. to ### Step 3: for consistency.

### Step 3. Take the Certification Exam!