verify does not validate acceptable spec.maturity values

[eriknelson]

We had set our spec.maturity value to release, only to discover the installplan will fail to install based on the following error:

spec.maturity in body should be one of [planning pre-alpha alpha beta stable mature inactive deprecated]

Expected outcome:
operator-courier should fail a CSV with invalid maturity.

Actual outcome:
No failure:

# operator-courier verify ./mig-operator; echo $?  
0