Update reference to S/MIME, Code Signing BRs

[wthayer]

Section 1.1 includes the below paragraph which could be updated to acknowledge the existence of BRs for S/MIME and Code Signing:

These Requirements only address Certificates intended to be used for authenticating
servers accessible through the Internet. Similar requirements for code signing,
S/MIME, time-stamping, VoIP, IM, Web services, etc. may be covered in future
versions.

[github-actions]

This issue was created based on:

• TLS BR Version 2.2.2
• EVG Version 2.0.1