Remove breaking change for createUser in Chart

[jscheffl]

Pull requests #59767 #59762 broke the backwards compatibility and therefore violated against SemVer promise of Helm Chart.

Therefore as a fix, this PR restores the possibility to still (deprecated) define the user via webserver.defaultUser.

Note: The PR doe NOT restore the property apiServer.defaultUser as besides schema definition the property was not used and was broken anyway. Specifying this in a previous config had no effect. If in 1.19.0 somebody will specify it it will raise an error instead.

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