Update CHANGELOG for v0.21.1 release #1869

Job	Run time
Generate semantic release version using git-describe-semver	12s
Build oldstable cgo-mingw-w64 x64 image using Makefile	1m 50s
Build oldstable cgo-mingw-w64 x86 image using Makefile	1m 31s
Build oldstable Alpine x86 image using Makefile	1m 21s
Build oldstable Alpine x64 image using Makefile	55s
Build oldstable image using Makefile	1m 56s
Build stable Alpine x64 image using Makefile	54s
Build stable Alpine x86 image using Makefile	57s
Build stable cgo-mingw-w64 x64 image using Makefile	1m 31s
Build stable cgo-mingw-w64 x86 image using Makefile	2m 2s
Build stable mirror images using Makefile	32s
Build stable image using Makefile	1m 38s
Build unstable Alpine x64 image using Makefile	54s
Build unstable Alpine x86 image using Makefile	56s
Build unstable cgo-mingw-w64 x64 image using Makefile	1m 33s
Build unstable cgo-mingw-w64 x86 image using Makefile	1m 30s
Build unstable image using Makefile	2m 0s
	22m 12s

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