1.x: mark RxJavaPlugins.registerErrorHandler as deprecated

[bobvanderlinden]

This marks RxJavaPlugins#registerErrorHandler and RxJavaPlugins#getErrorHandler as deprecated and documents RxJavaHooks#setOnError and RxJavaHooks#getOnError as its alternatives.

Related to #4566

[codecov-io]

Current coverage is 84.44% (diff: 100%)

Merging #4569 into 1.x will decrease coverage by 0.09%

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- Hits          14855      14838    -17   
- Misses         1864       1870     +6   
- Partials        852        863    +11   

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[akarnokd]

There are legitime uses of the old hooks so I'm not certain they should be deprecated.

[bobvanderlinden]

@akarnokd Ah, I gathered from #4566 and from RxJavaPlugins#getInstance and RxJavaErrorHandler#handleError being deprecated that RxJavaPlugins#registerErrorHandler would be deprecated as well.

This PR was mostly for people running into the same problem as me: using RxJavaPlugins#getInstance and implementing RxJavaErrorHandler#handleError both giving deprecation warnings while it is unclear what to use instead. The deprecation of RxJavaPlugins#getInstance does refer to RxJavaHooks, but not specifically setOnError, like what was suggested in #4566.

[bobvanderlinden]

Most purposes of this PR are already solved by updating the documentation. See #4566.