[v17] Use second_factor in the v17 teleport-cluster chart

[espadolini]

Using the second_factors field in the cluster-auth-preference of a v17 cluster that still has v16 agents or clients can result in widespread problems due to our overly strict checks in (*AuthPreferenceV2).CheckAndSetDefaults. This PR reverts the change in default values in the teleport-cluster chart done in #53319, since the problem can be avoided just by not using the new field in favor of the old one.

This PR is straight for v17 since v18 will only have to deal with v17+ clients which will all support the new field correctly.

changelog: reverted the default behavior of the teleport-cluster Helm chart to use authentication.secondFactor rather than authentication.secondFactors to avoid incompatibility during upgrades

[github-actions]

Amplify deployment status

Branch	Commit	Job ID	Status	Preview	Updated (UTC)
espadolini/v17-second-factor-chart	HEAD	1	✅SUCCEED	espadolini-v17-second-factor-chart	2025-05-12 21:27:23

[hugoShaka]

Can we also update the teleport.yaml reference to say something like "DO NOT USE FEATURE UNTIL ALL YOUR AGENTS ARE v17 ELSE YOUR CLUSTER WILL EXPLODE" because not every user deploy with Helm.

[espadolini]

Can we also update the teleport.yaml reference

@Joerger said he's gonna address the docs around second_factors more broadly.