Fix date filter for publikationen

[k-nut]

The current list of publications is broken since the hugo update.

The reason for this is that in publikationen.html layout, we use GroupByDate.
In the upgrade, we have renamed a lot of fields to publishedDate. In our config, we set

[frontmatter]
date  = [":filename", ":default"]

instructing hugo to first look at the filename when trying to find a date, and then use the default list. This list notably doesn't include any field called publishedDate, only publishDate is listed.
This led to the filters only taking into account the dates from the file names and no longer from the front matter. Since only three fields had valid dates, only those would be considered for the date filter.

This change hence renames all the dates field in the publikationen pages to date again to be consistent with how hugo usually does things.

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