Update CHANGELOG for v0.21.5 release #1911

Job	Run time
Generate semantic release version using git-describe-semver	7s
Build stable image using Makefile	1m 47s
Build stable Alpine x64 image using Makefile	54s
Build oldstable Alpine x86 image using Makefile	58s
Build oldstable Alpine x64 image using Makefile	54s
Build oldstable cgo-mingw-w64 x86 image using Makefile	1m 34s
Build oldstable cgo-mingw-w64 x64 image using Makefile	1m 29s
Build stable Alpine x86 image using Makefile	55s
Build stable cgo-mingw-w64 x64 image using Makefile	1m 30s
Build stable cgo-mingw-w64 x86 image using Makefile	1m 38s
Build unstable Alpine x64 image using Makefile	58s
Build unstable Alpine x86 image using Makefile	59s
Build unstable cgo-mingw-w64 x64 image using Makefile	1m 40s
Build oldstable image using Makefile	1m 40s
Build stable mirror images using Makefile	57s
Build unstable cgo-mingw-w64 x86 image using Makefile	1m 36s
Build unstable image using Makefile	2m 5s
	21m 41s

Provide feedback