[WARNING] Compiler Warnings When Targetting .Net 8+ (Please Merge #611)

[uniblab]

** Issue **
New compiler warnings are generated when compiling under .Net 8+. This is because the many Exception-derived classes in the BouncyCastle.Cryptography library implement or override code which has been marked Obsolete in the .Net 8+ runtimes. As a best practice, compiler warnings should be resolved instead of ignored.

** Solution **
Mark each such method with the Obsolete attribute and then Molly-guard the attribute inside NET8_0_OR_GREATER. This way it does not alter current code but ensures proper .Net 8+ compilation. This solution has already been implemented and made available as Pull Request #611 .

** Requested Action **
Please review and merge PR #611 . This PR has passed all tests on all configurations.

** Impact **
There is no impact to current compilation settings. It resolves numerous compiler warnings when compiling under .Net 8+, which is expected to happen in the near future.