[release/9.0-staging] Don't expose TrustedCertificatesDirectory() and StartNewTlsSessionContext() to NetFx

[github-actions]

Backport of #113963 to release/9.0-staging

/cc @steveharter

v8 backport: #115008

Customer Impact

• Customer reported
• Found internally

Two recently added members are now removed from the NetStandard package so that a project targeting NetStandard will get a compile error if they try to use the members instead of a MissingMethodException if the members are called when running under .NET Framework.

Regression

• Yes
• No

Testing

Verified the NetStandard2.0 package does not contain:

• LdapSessionOptions::get_TrustedCertificatesDirectory()
• LdapSessionOptions::set_TrustedCertificatesDirectory()
• LdapSessionOptions::StartNewTlsSessionContext()

Risk

Low. The method does not exist in the .NET Framework implementation, so we don't expose it in NetStandard.

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

[ghost]

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, 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]

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, 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. 

[dotnet-policy-service]

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[steveharter]

/ba-g unrelated System.Globalization.Tests failures