x/telemetry/internal/mmap: fails to build on GOOS=solaris GOARCH=amd64 on Go 1.19 release branch

[dmitshur]

From https://build.golang.org/log/e20b219bc7acca61048578e435da438c8e05d0e3:

# golang.org/x/telemetry/internal/mmap
internal/mmap/mmap_unix.go:31:23: undefined: syscall.Mmap
internal/mmap/mmap_unix.go:42:17: undefined: syscall.Munmap
note: module requires Go 1.20

Given go.mod says "1.20", maybe Go 1.19 shouldn't be attempted to be supported, and the builder just needs to require at least Go 1.20 branch.

CC @golang/solaris, @rorth.

[gopherbot]

Change https://go.dev/cl/505580 mentions this issue: internal/counter: skip tests on solaris, android, and GOARCH=386

[gopherbot]

Change https://go.dev/cl/505715 mentions this issue: internal/mmap: use syscall.Mmap on solaris only with go1.20+