Update release notes, etc., for the 1.47.3 release #214

Job	Run time
Build and test with gcc	1m 3s
Build and test with clang	2m 31s
Build and test with UBSAN enabled	2m 28s
Build mke2fs on Windows with mingw32	2m 19s
Build mke2fs on Windows with mingw64	2m 4s
Cross-compile for Android (aarch64)	37s
Build and test on macOS	1m 20s
Cross-compile for Android (armv7a)	59s
Build and test with gcc -m32	1m 29s
Cross-compile for Android (i686)	49s
Cross-compile for Android (x86_64)	43s
Build and test with ASAN enabled	3m 5s
	19m 27s

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