Add a default to Cloud Functions runtime

[rileykarson]

Fixes an issue where the Cloud Functions API now requires a runtime to be explicitly defined.

All existing provider versions are affected;

• 0.X, 1.X series providers have no workaround. Creation of new Cloud Functions is immediately broken.

• 2.X series providers up to 2.6.0 can work around the change by specifying a runtime.

This is a mostly-safe transition, to the point that we can be comfortable including this in a bugfix release.

Prior to today, the API has defaulted to nodejs6 as the runtime for Cloud Functions. That has been exposed in the runtime variable in the API response, so all existing functions will have it set in state. They will see no diff. Users who explicitly defined a runtime are unaffected.

There is one exception- if users have defined a function in Terraform without specifying runtime and then modified the function out of band (or defined a runtime then removed it from config, due to how Computed works) they will see a diff. That said, it will be caught at plan time and we will highlight it in the changelog so anyone running Terraform in CI will be warned.

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[all]

[terraform]

[terraform-beta]

[ansible]

[inspec]

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
This PR seems not to have generated downstream PRs before, as of 6d89632.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#697
depends: hashicorp/terraform-provider-google#3605