Route user.logout.confirm has no default _title [#3605588]

Problem/Motivation

With #144538: User logout is vulnerable to CSRF the route user.logout.confirm was introduced, but without a _title default value. In fact that this is a route to a form we should add a default _title.

Furthermore I was fixing my tests in #3605475: ETrackerFunctionalTests are not working and because of the logic in Drupal\Tests\UiHelperTrait::drupalLogout() the test gets the user.logout.confirm route/page in the session and the title_resolver service can't get a title from the route.

Proposed resolution

Add _title to user.logout.confirm

Issue fork drupal-3605588

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3605588-route-user.logout.confirm-has changes, plain diff MR !16127
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