Fixes https://github.com/spring-projects/spring-integration/issues/8713
* Introduce `DefaultSftpSessionFactory.createSftpClient()` factory method
which can be overridden for any custom `SftpClient` use-case
* Add changes to the docs
* Some code clean up
**Cherry-pick to `6.1.x`**
# Conflicts:
# spring-integration-sftp/src/main/java/org/springframework/integration/sftp/session/DefaultSftpSessionFactory.java
# src/reference/antora/modules/ROOT/pages/sftp/session-factory.adoc
# src/reference/antora/modules/ROOT/pages/whats-new.adoc
78b09ed610