x/net/idna: ToUnicode accepts Punycode labels encoding pure ASCII labels [1.25 backport]

[gopherbot]

@neild requested issue #78760 to be considered for backport to the next 1.25 minor release.

@gopherbot please create backport issues for this security fix.

[gopherbot]

Change https://go.dev/cl/798000 mentions this issue: [internal-branch.go1.25-vendor] idna: reject all-ASCII xn-- labels on all Go versions

[gopherbot]

Change https://go.dev/cl/799121 mentions this issue: [release-branch.go1.25] all: update x/net

[gopherbot]

Closed by merging CL 799121 (commit 2d5129d) to release-branch.go1.25.