Update CHANGELOG for v0.21.6 release #1926

Job	Run time
Generate semantic release version using git-describe-semver	10s
Build oldstable Alpine x64 image using Makefile	1m 10s
Build stable image using Makefile	1m 32s
Build stable Alpine x64 image using Makefile	50s
Build oldstable Alpine x86 image using Makefile	1m 1s
Build oldstable cgo-mingw-w64 x64 image using Makefile	1m 27s
Build oldstable cgo-mingw-w64 x86 image using Makefile	1m 30s
Build stable Alpine x86 image using Makefile	59s
Build stable cgo-mingw-w64 x64 image using Makefile	1m 36s
Build unstable Alpine x64 image using Makefile	53s
Build stable cgo-mingw-w64 x86 image using Makefile	1m 35s
Build unstable Alpine x86 image using Makefile	1m 0s
Build oldstable image using Makefile	1m 41s
Build stable mirror images using Makefile	29s
Build unstable cgo-mingw-w64 x64 image using Makefile	1m 29s
Build unstable cgo-mingw-w64 x86 image using Makefile	1m 48s
Build unstable image using Makefile	2m 1s
	21m 11s

Provide feedback