SVGs generated with blocklys screenshot functionality are kind of invalid

[BjoernLuig]

Describe the bug
SVGs generated with blocklys screenshot functionality are kind of invalid and can not be processed (properly) by some applications.

To Reproduce
Steps to reproduce the behavior:

1. Go to blockly
2. click on "download screenshot"

Show SVG in ubuntu file explorer

Upload SVG to Canva -> doesnt work

Download such an SVG from a Mattermost Message -> Mattermost crashes

Same issues seem to persist over multiple operating systems (Ubuntu, Windows, Mac). Same Programms (Browsers) seem to work around this problem. SVG files from other sources do not have these problems.

Expected behavior
Blocklys screenshot functionality creates a "valid SVG file.

Additional context
Some users had similar problems with invalid SVGs from other sources.
photopea/photopea#4663