[JENKINS-76295] Allow SSH build agents to connect (#11310)

[JENKINS-76295] Revert "Update dependency commons-io:commons-io to v2.21.0 (#11276)"

https://issues.jenkins.io/browse/JENKINS-76295 reports that SSH build
agents fail to connect after this change was included.

Testing done:

* Confirmed that Jenkins 2.537 fails to start an SSH build agent on
  Debian 13
* Confirmed that with this change reverted, the SSH build agent starts
  as expected on Debian 13

This reverts commit ab45729.

Author: MarkEWaite

bom/pom.xml

@@ -119,7 +119,7 @@ THE SOFTWARE.
       <dependency>
         <groupId>commons-io</groupId>
         <artifactId>commons-io</artifactId>
-        <version>2.21.0</version>
+        <version>2.20.0</version>
       </dependency>
       <dependency>
         <groupId>commons-lang</groupId>