Enable Puma by default in GDK

Problem

Puma is concurrent web server for Ruby. Puma is included in GitLab, but Puma support is experimental at this point in time. Enabling Puma by default should reduce the memory required by the GitLab application.

Proposal

GDK already supports Puma if explicitly enabled (!532 (merged) and !532 (merged)).

Change opt-in flag EXPERIMENTAL_PUMA to flag USE_WEB_SERVER. If USE_WEB_SERVER is undefined, use puma by default. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30285

GDK documentation updated accordingly: !712 (merged)