Improve documentation for handling expiring PIV/CAC certificates

[aduth]

🛠 Summary of changes

Updates documentation for handling expiring PIV/CAC certificates.

Notably:

• Updates the appdev on-call guide to reference the detailed article for troubleshooting expiring certificates
• Adds a reference to notifications for Federal PKI ecosystem changes, which often include postings informing about an upcoming replacement certificate
  • For example, a certificate expiring at the time of writing "Lockheed Martin Root Certification Authority 6" has a related posting advising "CertiPath intends to issue a new cross certificate to Lockheed Martin Root CA 6 on or about April 16, 2024" (LG-12998)
• Adds a reference to removing expired certificates when no replacements exist
  • For example, a certificate expiring at the time of writing "Exostar Federated Identity Service Root CA 2" has no replacements as is slated for removal (LG-12997)