Simplify the MultiSubscribe operator implementation

[jponge]

• Reuse more of the super class defaults
• Account for later cancellation

[codecov]

Codecov Report

Merging #1257 (c04f32a) into main (a44539f) will increase coverage by 0.14%.
The diff coverage is 78.57%.

❗ Current head c04f32a differs from pull request most recent head 74b435a. Consider uploading reports for the commit 74b435a to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1257      +/-   ##
============================================
+ Coverage     89.49%   89.63%   +0.14%     
- Complexity     3341     3349       +8     
============================================
  Files           458      458              
  Lines         13234    13231       -3     
  Branches       1650     1650              
============================================
+ Hits          11844    11860      +16     
+ Misses          721      716       -5     
+ Partials        669      655      -14     

Impacted Files	Coverage Δ
...rye/mutiny/operators/multi/MultiSubscribeOnOp.java	88.88% <78.57%> (+2.22%)	⬆️

... and 12 files with indirect coverage changes