[release/6.0] OpenSslX509ChainProcessor: ignore NotSignatureValid on last element.

[tmds]

Backport of #69668 to release/6.0

Fixes: #65874 (comment)

Customer Impact

RHEL9's default crypto policy no longer accepts the use of RSA+SHA1 signatures.

Because .NET uses strict OpenSSL validation, it does not accept certain certificates, which are considered valid by other tools (like curl/wget). This causes websites like https://pkgs.dev.azure.com to no longer be accessible using HttpClient on RHEL9.

This change relaxes the validation so these certificates are trusted by .NET.

Testing

New tests are included as part of the change.

Risk

Low. The existing tests, combined with the new tests, give confidence to the scoped change. The version in main (and the backport) were authored by a Red Hat employee, so we feel that the new RHEL9 scenario has gotten about as good an eye as it could get.

[tmds]

@bartonjs feel free to improve my initial message.

[tmds]

cc @omajid

[ghost]

Tagging subscribers to this area: @dotnet/area-system-security, @vcsjones
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Backport of #69668 to release/6.0

Fixes: #65874 (comment)

Customer Impact

RHEL9 default crypto policy no longer accepts the use of RSA+SHA1 signatures.

Because .NET uses strict OpenSSL validation, it does not accept certain certificates, which are considered valid by other tools (like curl/wget). This causes websites like https://pkgs.dev.azure.com to no longer be accessible using HttpClient on RHEL9.

This change relaxes the validation so these certificates are trusted by .NET.

@bartonjs @vcsjones ptal.

Author:	tmds
Assignees:	-
Labels:	area-System.Security
Milestone:	-

[bartonjs]

The failed legs are Helix warnings about queue deprecation, no tests failed.

[carlossanlop]

Servicing-approved label applied. Area owner signed off. CI passed with unrelated failures.