[release/9.0-staging] Replace a few SuppressMessage annotations with UnconditionalSuppressMessage

[github-actions]

Backport of #109180 to release/9.0

/cc @eiriktsarpalis

Customer Impact

• Customer reported
• Found internally

Fixes a customer reported regression from MAUI that manifests as as false positive linker warning when trimming a System.Text.Json application.

Regression

• Yes
• No

Introduced by #105032. A linker suppression erroneously used SuppressMessageAttribute instead of UnconditionalSuppressMessageAttribute resulting in the warning being suppressed at build time but not at publish time.

Testing

Manually verified that a warning no longer emanates from the impacted member.

Risk

Low. Makes a trivial change to a linker annotation.

[dotnet-policy-service]

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

[jeffhandley]

This will be queued up for 9.0 servicing once the release/9.0-staging branch is ready.

[carlossanlop]

@eiriktsarpalis - Please retarget this PR to the release/9.0-staging branch.

You can retarget to that branch by clicking on the Edit button on the top right (next to the PR title) and choosing the release/9.0-staging branch from the dropdown. Important: Please make sure you don't bring any unrelated changes from the release/9.0 branch when retargeting to release/9.0-staging.

[eiriktsarpalis]

Done.

[jeffhandley]

@artl93 This is ready for Servicing now

[jeffhandley]

@carlossanlop I just realized this is still sitting open; it needs your approval and then we can merge it for February servicing.