[APM] react-syntax-highlighter has started to treat linebreaks differently

[sorenlouv]

After updating react-syntax-highlighter in #84296 was necessary to modify the linebreak behaviour: 2be72fa

We should do two things:

• Look into why this was necessary and if there is a better solution
• Add a test that will catch changes like this in the future

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[smith]

EuiCodeBlock supports line numbers as of elastic/eui#4993, so we can replace the code we're using with that now. Closing this.