Adapt to dialog changes in matrix-auth 3.2.3

[daniel-beck]

Fixes #1803

Testing done

BROWSER=firefox JENKINS_VERSION=2.462.3 mvn test -Dtest=MatrixAuthPluginTest
BROWSER=firefox JENKINS_VERSION=2.462.3 VERSION_OVERRIDES=matrix-auth=3.2.2 mvn test -Dtest=MatrixAuthPluginTest
BROWSER=firefox JENKINS_VERSION=2.462.3 VERSION_OVERRIDES=matrix-auth=3.2.3 mvn test -Dtest=MatrixAuthPluginTest

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