Remove the outupdated unknown key/value pairs comment in CLUSTER SYNCSLOTS ESTABLISH

[enjoy-binbin]

We now have #2688 SYNCSLOTS CAPA, remove the outupdated comment.

[zuiderkwast]

We have SYNCSLOTS CAPA now: #2688. We don't use it now, but for the future it provides a way to send information where unknown information will be ignored by older nodes.

Do we still need this PR or can we close it?

[murphyjacob4]

Yeah I agree - let's use SYNCSLOTS CAPA - if anything unknown arrives outside of that - it would be unexpected. @enjoy-binbin wdyt?

[enjoy-binbin]

i normally like the ignore way in internal command, but i guess capa can work well as well, let's just remove the outupdated comment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.62%. Comparing base (54da834) to head (b8ad8cd).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2498      +/-   ##
============================================
- Coverage     72.64%   72.62%   -0.02%     
============================================
  Files           128      128              
  Lines         71278    71278              
============================================
- Hits          51777    51765      -12     
- Misses        19501    19513      +12     

Files with missing lines	Coverage Δ
src/cluster_migrateslots.c	92.07% <ø> (-0.16%)	⬇️

... and 15 files with indirect coverage changes

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