Updated to use the proper bouncycastle.

[chief-micco]

Support for modern dotnet was added years ago, and this is just catching us up to that

To help us get this pull request reviewed and merged quickly, please be sure to include the following items:

• Tests (if applicable)
• Documentation (if applicable)
• Changelog entry
• A full explanation here in the PR description of the work done

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Tests
• Other

Backward Compatibility

Is this change backward compatible with the most recently released version? Does it introduce changes which might change the user experience in any way? Does it alter the API in any way?

• Yes (backward compatible)
• No (breaking changes)

What's new?

• The bouncycastle legion has a package that supports modern dotnet now, so we can switch away from the fork we had been using since our project started.