Update CHANGELOG for v0.21.8 release #1968

Job	Run time
Generate semantic release version using git-describe-semver	5s
Build oldstable Alpine x86 image using Makefile	1m 11s
Build oldstable cgo-mingw-w64 x64 image using Makefile	1m 55s
Build oldstable Alpine x64 image using Makefile	1m 31s
Build oldstable cgo-mingw-w64 x86 image using Makefile	2m 49s
Build oldstable image using Makefile	2m 30s
Build stable Alpine x64 image using Makefile	1m 30s
Build stable Alpine x86 image using Makefile	1m 58s
Build stable cgo-mingw-w64 x64 image using Makefile	2m 14s
Build stable cgo-mingw-w64 x86 image using Makefile	1m 58s
Build stable image using Makefile	2m 9s
Build stable mirror images using Makefile	39s
Build unstable Alpine x64 image using Makefile	56s
Build unstable Alpine x86 image using Makefile	1m 2s
Build unstable cgo-mingw-w64 x64 image using Makefile	1m 50s
Build unstable cgo-mingw-w64 x86 image using Makefile	1m 37s
Build unstable image using Makefile	2m 36s
	28m 30s

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