thread-safe Random generation

[amiru3f]

Proposed Changes

As you may know, using Random.Shared is new-generation approach in net6 or upper, It's thread safe and optimized. So I replaced old _rnd.Next() with Shared Random if and only if the platform is net6 or later.

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Further Comments

[michaelklishin]

Thank you!

[michaelklishin]

@amiru3f for random endpoint selection within connection, I don't think we absolutely need to use a thread safe version but nothing wrong with using a modern .NET 6 option when we can 👍