add v1alpha1 diagnostic message / updated docs

[BBBmau]

Description

Prevents any future users to get issues when using helm provider by leading them to right version to use.

when running terraform plan with api_version set to v1alpha1 in exec block

│ Warning: v1alpha1 is not recommended, use v1beta1
│ 
│   with provider["registry.terraform.io/hashicorp/helm"],
│   on main.tf line 14, in provider "helm":
│   14:       api_version = "client.authentication.k8s.io/v1alpha1"
│ 
│ v1alpha1 authentication API was removed in Kubernetes 1.24, removal of alpha APIs is expected in minor version bumps of Kubernetes
│ 
│ (and one more similar warning elsewhere)

Release Note

Release note for CHANGELOG:

NONE

References

#893

Community Note

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