Clarify deprecation of EXTRA_QT_PLUGINS and update README.md #244

Job	Run time
aarch64 appimage	7m 22s
x86_64 coverage	5m 31s
armhf appimage	8m 21s
x86_64 appimage	6m 22s
i386 appimage	6m 13s
Create release and upload artifacts	7s
	33m 56s

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