These styles were going into an office:styles element in content.xml,
but this is invalid. Instead they must go in styles.xml. See #9287.
The variable `highlighting-styles` no longer has any effect on
the default opendocument template, and highlighting styles are
not included in opendocument output.
The only element in meta for which data/templates/default.opendocument
allows block element content appears to be "abstract", which is already
filtered out of meta' at this point, so simply convert all blocks to
linebreak-separated inlines.
Fixes: #8256
John MacFarlane