fix(certification): use camelCase for certificate fields

The identifier-naming migration requires camelCase wire format. Updated
snake_case YAML frontmatter keys to match the v1beta2 Certificate struct:
- expires_in → expiresIn
- issuing_authorities → issuingAuthorities
- signature_url → signatureUrl

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>
Signed-off-by: Raunak Madan <madanraunak24@gmail.com>

Author: CodexRaunak

content/certifications/c5ada327-8a58-4c8a-b9fa-51b95696488c/certified-meshery-contributor/_index.md

@@ -13,12 +13,12 @@ categories: "platform"
 
 certificate:
     title: "Certified Meshery Contributor"
-    expires_in: 24 # in months
+    expiresIn: 24 # in months
 
-    issuing_authorities:
+    issuingAuthorities:
       - name: "Meshery Authors"
         role: "Maintainers"
-        signature_url: "https://raw.githubusercontent.com/meshery-extensions/meshery-academy/6843c0074f73932274864aa68773ab605fd69ef3/static/c5ada327-8a58-4c8a-b9fa-51b95696488c/images/meshery-authors.png"
+        signatureUrl: "https://raw.githubusercontent.com/meshery-extensions/meshery-academy/6843c0074f73932274864aa68773ab605fd69ef3/static/c5ada327-8a58-4c8a-b9fa-51b95696488c/images/meshery-authors.png"
 
 # Table of content covered in the test
 # Each domain can have a weightage (percentage) and subdomains (items)