SVG Export: Write width and height in mm, not px

[bakajikara]

Resolves: #22531

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

[zacjansheski]

Tested on MacOS 14, Windows 11, Ubuntu 22.04.3. Approved
#22531 FIXED

[cbjeukendrup]

Awaiting approval from the MuseScore.com team, to be sure that this won't cause them issues

[cbjeukendrup]

Thanks!