crypto/x509: Reallow duplicate attributes in CSRs.

[robstradling]

Tracking https://go-review.googlesource.com/c/go/+/428636

If accepted, I'd like this to be considered for backport to the next 1.19 minor release.

[mknyszek]

CC @golang/security

[rolandshoemaker]

Fixed by http://go.dev/cl/428636.

[FiloSottile]

This was a fix for a regression introduced by #50988, and #57556 is the Go 1.19 backport request.

[gopherbot]

Change https://go.dev/cl/460236 mentions this issue: [release-branch.go1.19] crypto/x509: reallow duplicate attributes in CSRs