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

Checklist

• [x] I have read the contributing guidelines and signed the CLA.
• [x] I have referenced an issue containing the design document if my change introduces a new feature.
• [x] I have read the security policy.
• [x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got green light (please contact [email protected]) from the maintainers to push the changes.
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added necessary documentation within the code base (if appropriate).

[vinckr]

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