add Go 1.21 to the build matrix #185

Job	Run time
Coverage	33s
ycat	14s
Test with -race (ubuntu-latest, 1.18)	27s
Test with -race (ubuntu-latest, 1.19)	19s
Test with -race (ubuntu-latest, 1.20)	26s
Test in i386 (ubuntu-latest, 1.18)	25s
Test with -race (ubuntu-latest, 1.21)	33s
Test in i386 (ubuntu-latest, 1.19)	27s
Test with -race (macos-latest, 1.18)	47s
Test in i386 (ubuntu-latest, 1.20)	22s
Test with -race (macos-latest, 1.19)	30s
Test in i386 (ubuntu-latest, 1.21)	27s
Test in i386 (windows-latest, 1.18)	4m 23s
Test with -race (macos-latest, 1.20)	48s
Test in i386 (windows-latest, 1.19)	1m 35s
Test with -race (macos-latest, 1.21)	48s
Test in i386 (windows-latest, 1.20)	51s
Test with -race (windows-latest, 1.18)	2m 0s
Test in i386 (windows-latest, 1.21)	1m 49s
Test with -race (windows-latest, 1.19)	1m 1s
Test with -race (windows-latest, 1.20)	1m 23s
Test with -race (windows-latest, 1.21)	2m 11s
	22m 19s

Provide feedback