Bump org.apache.sshd:sshd-sftp to 2.15.0

[Swierkowski]

In what version(s) of Spring Integration are you seeing this issue?

org.springframework.integration:spring-integration-sftp:6.4.2

Describe the bug

The bug is in org.apache.sshd:sshd-sftp dependency and is descirbed here:
apache/mina-sshd#642

[artembilan]

Thank you for bringing this up!

According to our policy we cannot upgrade our point version to a new minor version of dependency.
So, we are moving to this new 2.15.0 in our new 6.5.0.
According to the last request in that MINA issue, we may have 2.14.1 eventually.
And that's where Dependabot will pick it up for our 6.4.x.