Rename diagrams.net integration to draw.io

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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