Update the allowed values of the `ClusterExtension.Spec.UpgradeConstraintPolicy` to be more descriptive

[everettraven]

For the ClusterExtension.Spec.UpgradeConstraintPolicy field, the “Enforce” and “Ignore” allowed values are not very descriptive of their behavior. In an effort to be more descriptive it is proposed that the following changes are made to the allowed value names:

• Enforce → CatalogProvided
• Ignore → SelfCertified

The GoDoc comment should be updated to include explicit information as to what each allowed value means when specified.

[skattoju]

/assign