x/net/idna: ToUnicode accepts Punycode labels encoding pure ASCII labels [1.26 backport]

[gopherbot]

@neild requested issue #78760 to be considered for backport to the next 1.26 minor release.

@gopherbot please create backport issues for this security fix.

[gopherbot]

Change https://go.dev/cl/797981 mentions this issue: [internal-branch.go1.26-vendor] idna: reject all-ASCII xn-- labels on all Go versions

[gopherbot]

Change https://go.dev/cl/798020 mentions this issue: [internal-branch.go1.26-vendor] idna: reject all-ASCII xn-- labels on all Go versions

[gopherbot]

Change https://go.dev/cl/799120 mentions this issue: [release-branch.go1.26] all: update x/net

[gopherbot]

Closed by merging CL 799120 (commit a42fec4) to release-branch.go1.26.