RFR: 8284105: Update security libraries to use sealed classes

[Sean Mullan]

On Fri, 8 Apr 2022 16:17:19 GMT, Weijun Wang <weijun at openjdk.org> wrote:

> It looks `KrbTgsRep.java`, `Krb5ProxyCredential.java`, `Builder.java`, `Vertex.java`, `Validator.java`, and `RSAKeyPairGenerator.java` can all be made package private.

Good point, although I would prefer to leave `Validator` as public for now until we are more sure of the compatibility risk as there have been external dependencies on it.

It may be useful to apply the `sealed` modifier to package-private classes, but for this RFE that is not the goal, so I will remove the `sealed` modifier from these classes (if applicable) when I make them package-private.

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PR: https://git.openjdk.java.net/jdk/pull/8165