2.x: fix repeatWhen and retryWhen signatures

[akarnokd]

Adjust the repeatWhen and retryWhen signatures to accept Publisher<?> on Single and Completable, matching Flowable's signature.

Reported in #5135.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (2.x@2a4b18e). Click here to learn what that means.
The diff coverage is n/a.

@@          Coverage Diff           @@
##             2.x    #5136   +/-   ##
======================================
  Coverage       ?   95.81%           
  Complexity     ?     5601           
======================================
  Files          ?      620           
  Lines          ?    39820           
  Branches       ?     5599           
======================================
  Hits           ?    38154           
  Misses         ?      672           
  Partials       ?      994

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	522 <ø> (?)
src/main/java/io/reactivex/Single.java	99.29% <ø> (ø)	131 <ø> (?)
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	112 <ø> (?)

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