Fix Sierra Trac query

[Ryan Schmidt]

Could someone with some Trac knowledge fix the query that shows the list of tickets on the Sierra Problems page:

https://trac.macports.org/wiki/SierraProblems

Now that we are using the "highsierra" keyword for macOS High Sierra problems, the SierraProblems page is including those in its search results, which isn't what we wanted, since we already have a High Sierra Problems page for that:

https://trac.macports.org/wiki/HighSierraProblems

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We already had this problem long ago with the Lion Problems page showing Mountain Lion tickets as well:

https://trac.macports.org/wiki/LionProblems

But we never fixed the problem, instead opting to never create a Mountain Lion Problems page.