README.md: update Discord channel instructions #19655

Job	Run time
LP64-WIP	4m 4s
build-windows (imx8 imx8x imx8m imx8ulp)	8m 5s
build-linux (mnfst, imx8 imx8x imx8m imx8ulp)	6m 20s
build-windows (mtl)	5m 32s
build-linux (mnfst, mtl)	4m 56s
manifest-check	53s
build-windows (tgl tgl-h)	6m 42s
build-linux (mnfst, lnl)	5m 10s
build-linux (mnfst, ptl)	4m 40s
build-windows (-d, mtl)	6m 5s
build-linux (mnfst, tgl tgl-h)	5m 36s
build-linux (zmain, imx8 imx8x imx8m imx8ulp)	6m 37s
build-linux (zmain, mtl)	4m 34s
build-linux (zmain, lnl)	4m 33s
build-linux (zmain, ptl)	4m 38s
build-linux (zmain, tgl tgl-h)	5m 27s
build-linux (-d, mnfst, mtl)	5m 1s
build-linux (mnfst, --all)	14m 12s
compare-linux-win	32s
	1h 43m 37s

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