BE: Use HTTP GET for basic auth

[Haarolean]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)
Fixes #4127
Spring does use HTTP GET for OAuth and HTTP POST for login_form for some reason.
Our frontend does use GET and the OAuth filter is hardcoded, so replacing basic to GET

LogoutPageGeneratingWebFilter
LogoutWebFilter

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