Add missing params to WhenEach overloads

[stephentoub]

These were approved and intended but likely missed due to coming in at approximately the same time that params span support was added.

[dotnet-issue-labeler]

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[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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[stephentoub]

/ba-g stuck leg