`generate_adhoc_ssl_pair()` uses hardcoded subject alternative name of `*`

[henryk]

Steps to reproduce:

• Use a development server created with werkzeug.service.generate_adhoc_ssl_pair(cn="localhost") (I'm using pytest-httpserver)
• Connect with Python built-in methods (making sure to provide the adhoc certificate as a CA file)

Actual results:

• A hardcoded SAN of * is added to the certificate, leading to ssl.SSLCertVerificationError: ("sole wildcard without additional labels are not support: '*'.",)

Expected results:

• The cn value should also be the single DNS name in the SAN

Environment:

• Python version: Python 3.8.5
• Werkzeug version: 2.0.1