Add app server type to usage ping

What does this MR do?

We are working on a migration from Unicorn to Puma. It would be helpful to have some data to track usage during these different phases:

• During opt-in, we can determine how much testing Puma has actually gotten to provide confidence going into opt-out phase
• During opt-out, we can track how many people are still on Unicorn, and we can make a more informed decision on when to remove it

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #208502 (closed)