LuaLaTeX and \pdfminorversion

As of LuaTeX v0.87, the pdfTeX-derived extension primitives are renamed. Here, you want

\pdfvariable minorversion

The details of emulation are given in the LuaTeX manual, or you can load the luatex85 package which wraps up this emulation for the transition.

You can also set the minor version on the Lua end.

\directlua{pdf.setminorversion(6)}
\documentclass{article}
\begin{document}
    This is a test
\end{document}

$ pdfinfo test.pdf 
Creator:        TeX
Producer:       LuaTeX-1.0.4
[...]
PDF version:    1.6

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