Fix POST method in dashboard link for disabling admin mode

What does this MR do?

Fix Leave Admin Mode button in admin mode, requiring a POST request since !22113 (merged).

There's two links in the dashboard, one for the normal view and another for smaller screen sizes. The normal one was still sending a GET request.

Also added a new spec to check both cases.

See also !22113 (comment 274121960)

🛠 with ❤ at Siemens

/cc @reprazent @ifarkas @rpaik @bufferoverflow

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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