encoding/xml: incompatible changes in the Go 1.21.0 [1.21 backport]

[gopherbot]

@bcmills requested issue #61881 to be considered for backport to the next 1.21 minor release.

@gopherbot, please backport to Go 1.21. Per #56986 this should at least have a release note and a GODEBUG guard.

[gopherbot]

Change https://go.dev/cl/522316 mentions this issue: [release-branch.go1.21] encoding/xml: overriding by empty namespace when no new name declaration

[gopherbot]

Closed by merging 2d07bb8 to release-branch.go1.21.