Obsolete some System.Security.Cryptography.Pkcs APIs

[vcsjones]

Contributes to #50528.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Tagging subscribers to this area: @bartonjs, @vcsjones, @krwq, @GrabYourPitchforks
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Issue Details

---

Contributes to #50528.

Author:	vcsjones
Assignees:	-
Labels:	area-System.Security, new-api-needs-documentation
Milestone:	-

[ghost]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[bartonjs]

Test failure is #39055. Merging.

[bartonjs]

Added to the checklist in dotnet/docs#23856