[backend] Redis hostname field should not be used when using cluster mode (#6280)

[aHenryJard]

Proposed changes

• Remove "hostname" field as mandatory on SSL configuration for cluster mode (it's "hostnames" that is mandatory in that mode)

Related issues

• fixes Redis in cluster mode requires overlapping hostname fields #6280

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case (coverage and e2e)
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.64%. Comparing base (8bbf3e8) to head (778694f).
Report is 44 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10978      +/-   ##
==========================================
+ Coverage   65.60%   65.64%   +0.04%     
==========================================
  Files         674      674              
  Lines       67138    67144       +6     
  Branches     7415     7421       +6     
==========================================
+ Hits        44046    44077      +31     
+ Misses      23092    23067      -25     

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