Upgrade to reactor 3.4.0

[piyushmor]

Spring Boot 2.4.0 uses project reactor version 3.4.0. We need to upgrade kotlinx-coroutines-reactor to use 3.4.0 version of project reactor for it to work with the new version of Spring Boot.

[qwwdfsad]

Unfortunately, we cannot do it right now because we are targeting Java 6, while Reactor 3.4.1 targets Java 8.
So we would like to postpone this update until we completely migrate to Java 8.

@sdeleuze do you have any objections against coroutines staying on 3.2.5.RELEASE?

[piyushmor]

hope the cost is worth it, we will not be able to support Spring Boot 2 then. Especially because there are a lot of breaking changes in reactor 3.4.0

[sdeleuze]

I don't think we have changed the Java requirements, as far as I remember Reactor is Java 8+ from the beginning so IMO it should be possible to do the upgrade on kotlinx-coroutines-reactor unless I missed something.

[qwwdfsad]

Aha, thanks! It seems like we were compiling this module against Java 6 and reactor didn't have the corresponding Gradle metadata prior to 3.4.1. We'll update it in the next release

[piyushmor]

I am not sure if this does not warrant an early release

[qwwdfsad]

Fixed with #2458