Clarify deprecation of EXTRA_QT_PLUGINS and update README.md #245

Job	Run time
aarch64 appimage	7m 19s
x86_64 coverage	6m 4s
armhf appimage	7m 25s
x86_64 appimage	5m 57s
i386 appimage	6m 42s
Create release and upload artifacts	34s
	34m 1s

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