all: drop Go 1.16 and Go 1.17 support

[hajimehoshi]

This is a meta issue to drop Go 1.16 and Go 1.17 supports.

The background is:

• gccgo12 already supports Go 1.18 (https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libgo/VERSION;h=39560f0fcdd63e4bb8eead6a3f849e91636d31eb;hb=2ee5e4300186a92ad73f1a1a64cb918dc76c8d67)
• Ubuntu stable already supports Go 1.18 (note that Debian stable supports Go 1.15 by default...)
• The Go team no longer supports Go 1.17 and older
• Semi-standard libraries like golang.org/x/sys/unix uses unsafe.Slice, which Go 1.16 doesn't support.

So, I decided to drop supporting Go 1.16 and Go 1.17.

Related:

• examples: remove example build tags when we can drop Go 1.16 #1462
• remove old // +build comments when we can drop Go 1.17 #2325
• all: use unsafe.Slice instead of reflect.SliceHeader when we can drop Go 1.16 #2420
• all: remove golang.org/x/sys/windows usages (or give up Go 1.16) #2419
• all: replace interface{}with any #2429

[hajimehoshi]

Thank you for your work, @mymmrac!