feat: support prod and staging GitHub Apps

[hanna-skryl]

This PR updates the authentication logic to support both the production Code PushUp GitHub App and the staging version, allowing the action to work regardless of which app users have installed.

[github-actions]

Code PushUp

🥳 Code PushUp report has improved – compared current commit 25e5743 with previous commit 20e83fc.

🕵️ See full comparison in Code PushUp portal 🔍

🏷️ Categories

🏷️ Category	⭐ Previous score	⭐ Current score	🔄 Score change
Code coverage	🟡 62	🟡 64
Bug prevention	🟢 100	🟢 100	–
Code style	🟢 100	🟢 100	–
Security	🔴 15	🔴 15	–
Updates	🟢 94	🟢 94	–

👍 1 group improved, 👍 3 audits improved

🗃️ Groups

🔌 Plugin	🗃️ Group	⭐ Previous score	⭐ Current score	🔄 Score change
Code coverage	Code coverage metrics	🟡 62	🟡 64

11 other groups are unchanged.

🛡️ Audits

🔌 Plugin	🛡️ Audit	📏 Previous value	📏 Current value	🔄 Value change
Code coverage	Branch coverage	🟥 42.2 %	🟥 46.7 %
Code coverage	Line coverage	🟨 61.7 %	🟨 65.8 %
Code coverage	Function coverage	🟨 71.9 %	🟨 72.7 %

169 other audits are unchanged.