Set net.core.somaxconn kernel parameter for Puma

Set net.core.somaxconn kernel parameter to a value equal or higher then the default Puma backlog_socket. The default kernel value is 128, which is lower then the default Puam backlog.

This was done for Unicorn in !405 (merged), but this was not done for Puma. Installations after GitLab 13.0+ might see lower throughput if this setting is not raised.

Closes #5752 (closed)