feat(net8): add .NET 8 target framework

[skwasjer]

• Adds .NET 8 target framework
• Uses new FrozenDictionary<,> in IbanRegistry
• Retested for performance regressions/improvements

The AppVeyor build will break as it is missing .NET 8 preview.

[codecov[bot]]

Codecov Report

Merging #180 (0b87860) into main (028cc3b) will increase coverage by 0.12%. The diff coverage is 100.00%.

Additional details and impacted files

| [Files](https://app.codecov.io/gh/skwasjer/IbanNet/pull/180?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martijn+Bodeman) | Coverage Δ | | |---|---|---| | [src/IbanNet/Registry/IbanRegistry.cs](https://app.codecov.io/gh/skwasjer/IbanNet/pull/180?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martijn+Bodeman#diff-c3JjL0liYW5OZXQvUmVnaXN0cnkvSWJhblJlZ2lzdHJ5LmNz) | `97.22% <100.00%> (+0.92%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/skwasjer/IbanNet/pull/180/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martijn+Bodeman)

[sonarcloud[bot]]

Quality Gate failed

Failed conditions

84.6% Coverage on New Code (required ≥ 85%)

See analysis details on SonarCloud