Add required_providers

[bharathkkb]

We should add required_providers and lock providers to > 3.38 to prevent errors from google_project_service_identity.

ref: terraform-google-modules/terraform-google-bastion-host#53, terraform-google-modules/terraform-google-log-export#60

[thiagonache]

We'll require >= 3.47 (probably) for the default service accounts resource. Should we wait to change it at once?

[morgante]

We'll require >= 3.47 (probably) for the default service accounts resource. Should we wait to change it at once?

Yes we can change it once as part of the next major release.

[bharathkkb]

fixed by #513