Implement agent header detection for .net8, .net9 etc

[sacOO7]

• Currently, we only detect .net6 and .net7 as a part of src/IO.Ably.NETStandard20/Platform.cs.
• We need to include detection for .net8, .net9 and .net10.
• This can also include detection for ios, android and other platforms ( optional ) as a part of .net frameworks
• Once those platforms detection is added, we need to update https://github.com/ably/ably-common/blob/main/protocol/agents.json accordingly.
• Also, update Agent.cs and src/IO.Ably.Tests.Shared/AgentTests.cs accordingly.

┆Issue is synchronized with this Jira Task by Unito