chore: replace Portable BC with BC Crypto

[texastony]

Issue #, if available:

Description of changes:
It does not appear that BouncyCastle is going to continue development of
Portable.BouncyCastle, which apparently is a labor of love/brilliance by one developer.

Instead, they seem to be committing to BouncyCastle.Cryptography.

See: https://www.bouncycastle.org/csharp/#DOWNLOAD20200

Squash/merge commit message, if applicable:
chore: replace Portable BC with BC Crypto

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Due to dafny-lang/dafny#2500, Traits are dangerous:

1. Does this PR add any traits or classes that extend a trait?
2. Are these traits annotated with {:termination false}?

The override checks on
the specifications on
a class' functions/methods/etc. validating
that specifications are
at least as strong as those on
the traits it implements
are not working correctly when
that trait is defined in a different module
(and hence must have {:termination false} on it).

As such, if either (1.) or (2.) is true:

• manually verified all the trait specifications are copied onto classes that extend them?

[texastony]

NVM, Package BouncyCastle.Cryptography 2.2.1 is not compatible with net452 (.NETFramework,Version=v4.5.2)