Rename diagrams.net integration to draw.io
The draw.io integration in GitLab is incorrect called diagrams.net. Per https://www.drawio.com/blog/move-diagrams-net and various other sources diagrams.net is just the reference implementation of the draw.io product.
Also need to update https://docs.gitlab.com/ee/administration/integration/diagrams_net.html, https://docs.gitlab.com/ee/user/markdown.html#diagramsnet-editor & various other docs
Edited by Ben Bodenmiller