Add option to set PDF version when exporting

[Philipp-p]

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Is your feature request related to a problem? Please describe.
When submitting a paper to certain conferences, they require you to limit your PDF version to a specific version. Draw.io gives you no option to set a PDF version when exporting.

Describe the solution you'd like
I would like to have an option in the export dialog to set a specific PDF version.

Describe alternatives you've considered
I tired to convert the obtain output pdf with Ghostscript, this proved successful.
An SVG export is of course also an option to avoid the issue, but I thought maybe an export option for the PDF version is something worth to include in the project.

[davidjgraph]

Looking at https://en.wikipedia.org/wiki/History_of_PDF PDF 1.7 came out in 2006 and is an ISO standard. What version is expected?

[Philipp-p]

They require PDF 1.5. I was also surprised when I found out how old this standard is…