[FEAT] Support More Build Targets (Please merge #610)

[uniblab]

Is your feature request related to a problem or specific use case? Please describe.
I am a direct consumer of SSH.NET and therefore a transitive consumer of BouncyCastle.Cryptography. There are improvements I'd like to make to my own programs which require supporting more SSH.NET build targets, which would require supporting more BouncyCastle.Cryptography build targets

Describe the solution you'd like
I have already submitted a Pull Request, #610 , which incorporates the additional build targets. It has passed all the included test cases in all configurations. I want PR #610 merged.

Describe alternatives you've considered
You could copy'n'paste my updates to the csproj files and issue your own PR, but it would be easier to just review and merge my PR.

Product deployment

• Increment version number to 2.6.0
• Update Nuget

Additional context
Changes consisted of adding a few more targets to the csproj files and then running the test cases. There are no code changes.

[peterdettman]

Motivation/justification for a PR belongs on the PR itself.