Update release notes, etc., for the 1.47.3 release #219

Job	Run time
Build and test with clang	1m 23s
Build and test on macOS	1m 16s
Build and test with gcc	59s
Build and test with UBSAN enabled	1m 35s
Build and test with ASAN enabled	3m 2s
Build mke2fs on Windows with mingw64	2m 5s
Build and test with gcc -m32	1m 25s
Build mke2fs on Windows with mingw32	1m 58s
Cross-compile for Android (x86_64)	34s
Cross-compile for Android (i686)	34s
Cross-compile for Android (armv7a)	32s
Cross-compile for Android (aarch64)	39s
	16m 2s

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