Replace `<br>` elements with `<?linebreak?>` processing instructions

[IanMayo]

DITA doesn't support the <br> element. We must provide the <?linebreak?> processing instruction instead: https://www.dita-ot.org/dev/reference/markdown/mdita-syntax#ariaid-title7

Find out how to do this with BS4.

Question posed here: https://stackoverflow.com/questions/77699002/generate-linebreak-in-dita-content

Note: here is how it is resolved in a different DITA plugin

[IanMayo]

This is working. It required an upgrade to OxygenXML