Removal of reference to Apache license and tidying up DCO

Removal of reference to the Apache 2.0: The existing text of the DCO states that the all contributions subject to the DCO are licensed under both the Apache 2.0 license and the MIT Expat license. This is due to a historic plan to move the GitLab codebase from the MIT Expat to the Apache 2.0. There are no longer plans to change the basis on which the GitLab codebase is licensed, so reference to the Apache 2.0 is longer required.

Formatting changes: The existing text of the DCO is formatted in such a way that the applicable project license was not clear. It also included a number of repetitive headings. Formatting has been corrected and these repetitive headings removed to make the DCO more readable for contributors.