SSH key generation vs. warnings

No, as you already mentioned, that is the expected behavior. You may have a look into the similar question about Module user WARNING:Found existing ssh key here.

I think the only option is to disable in Ansible Configuration Settings the ACTION_WARNINGS

By default, Ansible will issue a WARNING when received from a task action (module or action plugin). These warnings can be silenced by adjusting this setting to False.

by setting in example

grep 'defaults\|warnings' ansible.cfg
[defaults]
deprecation_warnings    = False
action_warnings         = False

and then resulting into an output of

TASK [ansible.builtin.user] ******************
ok: [localhost]

PLAY RECAP ***********************************
localhost                  : ok=1    changed=0
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