Fix OIDC Logout docs: Session Strategy vs. Registry

[hwanders]

Contents

This contains the following changes to the OIDC logout documentation:

• replace OidcSessionStrategy by OidcSessionRegistry
• replace ReactiveOidcSessionStrategy by ReactiveOidcSessionRegistry
• fix broken/inconsistent Java code examples
• fix syntax of adoc source code blocks -- same as in Fix code format in OIDC Logout docs #15566 to avoid conflicts if that one gets merged first

Furthermore a test name was adjusted to reflect the right terminology.

Reason

• There is no OidcSessionStrategy in Spring Security, the demo code indicates that actually OidcSessionRegistry was meant

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[jzheaux]

Thanks for the corrections, @hwanders! These are now merged into 6.2.x, 6.3.x, and main and should appear in the docs site shortly.