v5.5.4

Change log:

3bfabb9 Upgrade dependencies; prepare for release
726b216 Disable TcpNioConnectionTests.testCleanup()
3f6d205 Fix link in the channel-adapter.adoc
b92d52f Improve docs for global default poller
0fb64f1 Upgrade to Kotlin 1.5
0f2285d Fix reply producing to not block reactive thread
fe300eb GH-3614: JPA outbound components delete in batch (#3629)
25ac230 GH-3627: Fix race condition NPE in MqttPahoMDCA
4456caf Introduce high-level API for flows composition (#3624)
b8785e5 GH-3619: Fix Mail Receiver fetch redundant Messages
55876e0 GH-3620: Fix Shared SFTP Session