Use modern archiving and unarchiving methods in GTMAppAuthFetcherAuthorization.swift

[mdmathias]

Due to some archiving bugs, we had to revert back to the older methods for archiving and unarchiving GTMAppAuthFetcherAuthorization in Swift. This reversion should only be temporary as there is a fix underway. When ready, update the following:

• GTMAppAuthFetcherAuthorization.save(authorization:itemName:) to use NSKeyedArchiver.archivedData(withRootObject:requiringSecureCoding:)
• Uncomment the code checking for version availability (to call the modern unarchiving method) in GTMAppAuthFetcherAuthorizaiton.authorization(itemName:)

Both of these have commented out code and a link to this issue.

[petea]

This is closely related to #105.

[petea]

This has been addressed on master branch with #145 and bumping our App Auth min version to 1.6.0.

[mdmathias]

Closing per https://github.com/google/GTMAppAuth/pull/182#issue-1395194333