x/build: make >= go1.16 the bootstrap compiler for all builders

[martisch]

In Proposal #44505 it was accepted to make go1.16 the minimally required Go bootstrap version for go 1.18.

If we want to make the Go 1.18 release have cleanups that remove pre Go 1.16 work arounds in bootstrap and libraries we should upgrade the builders such that cleanup changes for Go 1.18 can still land while the tree is open.

I understand this is a large task as there are many builders so it might not work out in the end to have any changes that really need at least Go 1.16 as a bootstrap version in Go 1.18.

I made a cleanup CL for the sort package: https://go-review.googlesource.com/c/go/+/344351
Unfortunately all of the builders except linux-arm-aws (uses go1.15) failed as they seem to use an older (mostly go1.4) boostrap compiler.

/cc @dmitshur @cagedmantis

[martisch]

Closing as it is unlikely go1.16 will be made the bootstrap version for go1.18+.

Context: #44505 (comment)