When hand-coding html and line breaks are entered, line breaks are rendered as line breaks by the 'Covert line breaks' filter in core unlike in the 3.x branch

Clarified the summary to reflect the scenario; need to bear in mind that, while I do not have any hard data, I expect the vast majority of people are:

1. Using CK Editor
2. Using the footnotes module for footnotes rather than call-out boxed text type content

Arguably in 3.x the user should have expected line breaks to be converted to line breaks, but anyways, useful if someone comes across this: the easiest way to address in case you also use the above scenario hand-coding your html and adding line breaks within footnotes, you can strip line breaks for example in twig since footnotes are output in twig templates. Eg, trim accepts arguments for example.