[release/7.0-preview7] Mark JsonSerializerOptions.TypeInfoResolver as nullable and linker-safe

[github-actions]

Backport of #72044 to release/7.0-preview7

/cc @eiriktsarpalis

Customer Impact

This is a last-minute update to the nullability/linkability semantics of a key property in the new System.Text.Json contract customization feature that barely missed the snap deadline. We want to make sure that the change gets appropriate soak time by being incorporated in Preview 7. See #71960 for more details on the technical reasons that motivated the change.

Testing

Tests have been updated to reflect the changed semantics where appropriate.

Risk

Low. Changing semantics of a pre-release feature.

[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, 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-text-json, @gregsdennis
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Issue Details

---

Backport of #72044 to release/7.0-preview7

/cc @eiriktsarpalis

Customer Impact

Testing

Risk

IMPORTANT: If this change touches code that ships in a NuGet package, please make certain that you have added any necessary package authoring and gotten it explicitly reviewed.

Author:	github-actions[bot]
Assignees:	-
Labels:	area-System.Text.Json, new-api-needs-documentation
Milestone:	-

[carlossanlop]

Approved for servicing via email. Merging.