Obsolete key blob support on ECDiffieHellmanPublicKey

[vcsjones]

Obsoletes:

• M:System.Security.Cryptography.ECDiffieHellmanPublicKey.#ctor(System.Byte[])
• M:System.Security.Cryptography.ECDiffieHellmanPublicKey.ToByteArray()

Closes #50528 (Last one!)

[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.

[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: @dotnet/area-system-security, @vcsjones
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Issue Details

---

Obsoletes:

• M:System.Security.Cryptography.ECDiffieHellmanPublicKey.#ctor(System.Byte[])
• M:System.Security.Cryptography.ECDiffieHellmanPublicKey.ToByteArray()

Closes #50528 (Last one!)

Author:	vcsjones
Assignees:	-
Labels:	area-System.Security, breaking-change
Milestone:	-

[bartonjs]

Paperwork opened at dotnet/docs#28861.