balancer/ringhash: Add experimental notice in package comment

[arjan-bal]

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.31%. Comparing base (32e57de) to head (e5acb31).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8364      +/-   ##
==========================================
- Coverage   82.35%   82.31%   -0.04%     
==========================================
  Files         419      419              
  Lines       42034    42039       +5     
==========================================
- Hits        34615    34605      -10     
- Misses       5968     5977       +9     
- Partials     1451     1457       +6     

Files with missing lines	Coverage Δ
balancer/ringhash/ringhash.go	94.22% <ø> (ø)

... and 20 files with indirect coverage changes

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