[Java] Update ApiClient.mustache for retrofit2 + rxJava3

[PeregrineFalcon]

• Support to use official square implementation of rxJava3 adapter instead of 3rd party in code gen for retrofit and RxJava3
• Updates README to reflect that rxJava2 and rxJava3 are not actually deprecated

@martin-mfg for review since changes to the Java code gen

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• Read the contribution guidelines.
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  Commit all changed files.
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[wing328]

Updates README to reflect that rxJava2 and rxJava3 are not actually deprecated

we will decommission rxJava2 support as users should be using rxJava3 instead.

[wing328]

thanks for the PR

looks good to me and will merge after all tests pass.

[PeregrineFalcon]

Updates README to reflect that rxJava2 and rxJava3 are not actually deprecated

we will decommission rxJava2 support as users should be using rxJava3 instead.

Yeah I figured, that does make sense. I didn't want to make that change in this PR because it seemed drastic. Agreed rxJava3 should be the path going forward.