Add Release Note about the fix for dnstap output file rolling

@vicky wrote in !21 (merged):

There is no release note about this in the 9.12.1 RC - can we please mention that this is fixed? I think there were users who noticed it was broken in 9.12.0 who will be wondering if it is now fixed. https://bugs.isc.org/Public/Bug/Display.html?id=46942