Roster migrations, Cores now records

Added

• Feat: Add support for roster migrations (#131).

Changed

• deps: Use .NET 5 SDK for build, target only .NET 5 with NuGet packages.
• Fix: RosterTag name in v2.03 XSD schema (was tags, is tag) (#121).
• Refactor: Core types are now C# 9 nominal records (#125).
• Refactor: Xml serializers are now manually crafted using C# 9/.NET 5 Source Generators;
  this allows Xml serialization to work with C#9 records and (primarily) ImmutableArrays,
  as well as greatly reduce warmup-time in deployments like Blazor WASM, and remove
  any Reflection from that process (#130).