feat!: add deprecating message for the encrypted package

Description of the changes being introduced by the pull request:

The following PR adds a message saying that the encrypted package from go-tuf is already moved to https://github.com/secure-systems-lab/go-securesystemslib and will be deprecated here.

Existing dependants should use github.com/secure-systems-lab/go-securesystemslib/encrypted instead.

The PR also includes updating that dependency for go-tuf as well.

Release Notes:

Deprecated: The encrypted package from go-tuf is already moved to https://github.com/secure-systems-lab/go-securesystemslib and will be deprecated here. Use github.com/secure-systems-lab/go-securesystemslib/encrypted instead.

Types of changes:

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[x] Breaking change (fix or feature that would cause existing functionality to not work as expected). Please ensure that your PR title is a Conventional Commit breaking change (with a !, as in feat!: change foo).

Please verify and check that the pull request fulfills the following requirements:

theupdateframework / go-tuf