Use dotnet 7 regex source generator

[dashkan]

This PR adds support for dotnet 7 regular expression source generators.

Adds significant performance boost.

Since source generators require partial classes, I moved all regular expressions into a single file and wrap the partial methods with a property for simplicity.

I also upgraded all referenced nuget packages to latest.

dotnet 5 is not supported on Apple silicon. I had to also upgrade to dotnet 6.

dotnet 6 benchmark (Not using regex source generator)

// * Summary *

BenchmarkDotNet=v0.13.5, OS=macOS Ventura 13.2.1 (22D68) [Darwin 22.3.0]
Apple M1 Max, 1 CPU, 10 logical and 10 physical cores
.NET SDK=7.0.200
  [Host]     : .NET 6.0.14 (6.0.1423.7309), Arm64 RyuJIT AdvSIMD
  DefaultJob : .NET 6.0.14 (6.0.1423.7309), Arm64 RyuJIT AdvSIMD


|                                                          Method |     Mean |    Error |   StdDev |
|---------------------------------------------------------------- |---------:|---------:|---------:|
|                                                  SingleThreaded | 36.55 ms | 0.712 ms | 0.666 ms |
|                                         ParallelUponDeserialize | 38.59 ms | 0.762 ms | 1.068 ms |
|                                      ParallelUponGetOccurrences | 39.84 ms | 0.700 ms | 0.719 ms |
| ParallelDeserializeSequentialGatherEventsParallelGetOccurrences | 39.17 ms | 0.769 ms | 1.078 ms |

dotnet 7 benchmark (Using regex source generators)

// * Summary *

BenchmarkDotNet=v0.13.5, OS=macOS Ventura 13.2.1 (22D68) [Darwin 22.3.0]
Apple M1 Max, 1 CPU, 10 logical and 10 physical cores
.NET SDK=7.0.200
  [Host]     : .NET 7.0.3 (7.0.323.6910), Arm64 RyuJIT AdvSIMD
  DefaultJob : .NET 7.0.3 (7.0.323.6910), Arm64 RyuJIT AdvSIMD


|                                                          Method |     Mean |    Error |   StdDev |
|---------------------------------------------------------------- |---------:|---------:|---------:|
|                                                  SingleThreaded | 28.01 ms | 0.125 ms | 0.098 ms |
|                                         ParallelUponDeserialize | 33.15 ms | 0.631 ms | 0.751 ms |
|                                      ParallelUponGetOccurrences | 31.07 ms | 0.522 ms | 0.488 ms |
| ParallelDeserializeSequentialGatherEventsParallelGetOccurrences | 32.36 ms | 0.620 ms | 0.806 ms |

[axunonb]

@dashkan As you may have noticed we'd like to revitalize ical.net. Appreciate your PR, which looks good. AFAIK, source-generated regular expressions are supported for netstamdard2.1 and later, but they are not supported in .NET Framework. As I read the PR code Ical.Net.Utility already uses conditional compilation. So we could support all currently supported target frameworks, instead of restricting <TargetFrameworks>net6.0;net8.0</TargetFrameworks>. Does this make sense to you?