docs: back-channel logout client TLS configuration

[aarmam]

Back-Channel Logout client TLS configuration documentation

client:
  default:
    min_version: 
    max_version: 
    cipher_suites:
  back_channel_logout:
    min_version: 
    max_version: 
    cipher_suites:

Related Issue or Design Document

#2875

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  introduces a new feature.
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  appropriate).

[vinckr]

added the upstream label to make clear this is waiting for the PR in hydra.