Retire SHOW_LEGACY_TOKEN_TO_ADMINS system property in ApiTokenProperty

[Heller07]

Fixes #26377
Removes the deprecated SHOW_LEGACY_TOKEN_TO_ADMINS system property from ApiTokenProperty.
Since the permission changes introduced after JEP-223, administrators inherently have full control and can perform actions such as running Groovy scripts. Therefore limiting what administrators can do through this property no longer makes sense.
This change removes:

• the SHOW_LEGACY_TOKEN_TO_ADMINS system property
• the related SystemProperties usage
• the associated documentation block

Testing done

This change is a code cleanup that removes an unused system property and related configuration logic.
Testing performed:

• Built Jenkins locally using: mvn -DskipTests clean install
• Verified that the project compiles successfully after the removal.
• Reviewed the permission logic to ensure behavior remains unchanged: administrators continue to have access as before.

Screenshots (UI changes only)

N/A

Before

After

Proposed changelog entries

• Remove deprecated SHOW_LEGACY_TOKEN_TO_ADMINS system property from ApiTokenProperty

Proposed changelog category

/label internal

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue, if it exists, is well-described.
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• There is automated testing or an explanation as to why this change has no tests.
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Desired reviewers

@jenkinsci/core-pr-reviewers

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
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• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
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[MarkEWaite]

Needs a matching pull request to Jenkins documentation (here and here) to note that the properties have been removed

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[Heller07]

Hi @MarkEWaite I want to confirm the intended behavior before updating the tests.
Since the issue says the admin restriction code can be "deleted without replacement", I interpreted it as allowing administrators to see/manage tokens like other operations they can perform. I updated canCurrentUserControlObject accordingly.
However, some existing tests (e.g., adminsShouldBeUnableToSeeTokensByDefault) assert that admins cannot see tokens. Should these tests be removed/updated to reflect the new behavior, or should the original restriction remain and only the system property be removed?
Just want to make sure I align the implementation and tests with the intended behavior.