[release/9.0] Fix constraint on FrozenSet.{Try}GetAlternateLookup

[github-actions]

Backport of #107846 to release/9.0
Fixes #107840 (comment)

/cc @stephentoub

Customer Impact

• Customer reported
• Found internally

Regression

• Yes
• No

Testing

n/a

Risk

Low. It's just adding the additional anti-constraint in the ref assembly.

@carlossanlop, does this require any special authoring? It has a nuget package, but we also haven't shipped 9.0 GA yet.

[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, 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-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, 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.

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[eiriktsarpalis]

Might need to check if this change has any ramifications on API docs authoring.

[stephentoub]

/ba-g all failures are known

[carlossanlop]

This is also in main. Once we update the ref assemblies for dotnet-api-docs with the RC2 ones, the docs structure will get updated and it will match.

[carlossanlop]

In other words, no action is needed to address the docs concern.

[stephentoub]

In other words, no action is needed to address the docs concern.

Same for packaging, right?

[carlossanlop]

Same for packaging, right?

No package authoring changes needed for OOBs yet. We begin those when we start servicing this branch.