Use enhanced-for cycle instead of Enumeration in java.security.jgss

java.util.Enumeration is a legacy interface from java 1.0.
There are a few places with cycles which use it to iterate over collections. We can replace this manual cycle with enchanced-for, which is shorter and easier to read.

sun.security.jgss.ProviderList#addAllMechsFromProvider
sun.security.jgss.GSSCredentialImpl#dispose
sun.security.jgss.GSSCredentialImpl#getRemainingLifetime
sun.security.jgss.GSSCredentialImpl#getUsage()
sun.security.jgss.GSSCredentialImpl#getElements