crypto/x509: ignore Common Name by default

crypto/x509: ignore Common Name by default

Common Name has been deprecated for 20 years, and has horrible
interactions with Name Constraints. The browsers managed to drop it last
year, let's try flicking the switch to disabled by default.

Return helpful errors for things that would get unbroken by flipping the
switch back with the environment variable.

Had to refresh a test certificate that was too old to have SANs.

Updates #24151

Change-Id: I2ab78577fd936ba67969d3417284dbe46e4ae02f
Reviewed-on: https://go-review.googlesource.com/c/go/+/231379
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>