[Fleet+Elastic Agent] Clarify that upgrade rollbacks are not ready for use yet in relevant changelog entries (#2583)

The wording of the rollback changelogs is great but they imply to some
users that the feature is ready for use yet when it isn't. Add language
indicating these changes are building towards the feature in a future
release.

Co-authored-by: Visha Angelova <[email protected]>

Author: cmacknz

release-notes/fleet-elastic-agent/index.md

@@ -54,12 +54,8 @@ There are no new features, enhancements, or fixes in this release.
 
 **Elastic Agent**
 
-* Adds a new configuration setting, `agent.upgrade.rollback.window`. [#8065]({{agent-pull}}8065) [#6881]({{agent-issue}}6881)
+* Adds a new configuration setting, `agent.upgrade.rollback.window` in preparation for enabling upgrade rollbacks in a future release. [#8065]({{agent-pull}}8065) [#6881]({{agent-issue}}6881)
 
-  The value of the `agent.upgrade.rollback.window` setting determines the period after upgrading
-  Elastic Agent when a rollback to the previous version can be triggered. This is an optional
-  setting, with a default value of `168h` (7 days). The value can be any string that is parseable
-  by <https://pkg.go.dev/time#ParseDuration>.
 * Removes resource/k8s processor and uses k8sattributes processor for service attributes. [#8599]({{agent-pull}}8599)
 
   This PR removes the `resource/k8s` processor in honour of the k8sattributes processor that
@@ -93,7 +89,7 @@ There are no new features, enhancements, or fixes in this release.
 * Makes pbkdf2 settings validation FIPS compliant. [#4542]({{fleet-server-pull}}4542)
 * Updates to Go v1.24.0. [#4543]({{fleet-server-pull}}4543)
 * Adds version metadata to version command output. [#4820]({{fleet-server-pull}}4820)
-* Adds rollback attribute to upgrade actions. [#4838]({{fleet-server-issue}}4838)
+* Adds rollback attribute to upgrade actions in preparation for enabling upgrade rollbacks in a future release. [#4838]({{fleet-server-issue}}4838)
 
 ### Fixes [fleet-elastic-agent-9.1.0-fixes]