dl: switch to go install path@latest syntax in documentation

[dmitshur]

Now that Go 1.17 is released and Go 1.15 in unsupported per release policy, we should update documentation of golang.org/dl/go1.N commands to suggest the new go install path@latest installation syntax instead of go get per #43684.

It should be okay to update documentation for all golang.org/dl/... commands, even pre-1.15 ones, since they're still meant to serve supported Go versions, all of which by now understand the new syntax.

CC @golang/release, @bcmills, @jayconrod, @matloob.

[gopherbot]

Change https://golang.org/cl/346091 mentions this issue: all: switch to go install path@latest syntax in documentation

[gopherbot]

Change https://golang.org/cl/353430 mentions this issue: go.dev/_content/blog: update build constraints in fuzz-beta post