Description of the issue

An unclear or confusing error message appears in the "Update Cluster" modal when the user attempts to proceed without selecting a version.

**

If no version is selected during the upgrade process, the following error message is displayed (likely originating from the backend):

" problem occurred with that selected version."
"Missing mandatory field 'version'."

This message is misleading because the user hasn't selected any version yet. Referring to a "selected version" implies that the issue lies with a specific choice, which is not the case. Additionally, the phrase "mandatory field 'version" could be more user-friendly and better aligned with UI language expectations. 

Reproduction Steps:

1. Install a ROSA hosted cluster.
   Note: Use a version that supports Y-stream upgrades. For example, install version 4.17.37, which allows an upgrade to 4.18.
2. Once cluster is ready, Go cluster's Settings tab
3. Select the Update strategy as "Individual updates"
4. Click "Save" button.
5. Click "Update" button.
6. In "Update cluster" model, Do not select a version (i.e. unselect already selected version)
7. See the behavior.

Acceptance criteria

• The error message should clearly indicate that a version must be selected before proceeding.

• Suggested message (I believe we could consult UX for more accurate definition)

  "Please select a version to continue with the upgrade."