Use INumberBase<T>.CreateChecked on .NET 7+

[andrewimcclement]

Implementing change suggested in #1483 .

[lipchev]

I don't personally use these extensions, but here are a few more remarks:

• for the old framework it's possible to restrict the type to IConvertible and replace the Convert.ToDouble(value) with value.ToDouble(null) (given that we're already requiring a struct we don't have to concern ourselves with nullability)
• we're not testing the old framework:

  UnitsNet/UnitsNet.NumberExtensions.Tests/UnitsNet.NumberExtensions.Tests.csproj

  Line 4 in 8d4fa52

  <TargetFramework>net8.0</TargetFramework>

[andrewimcclement]

Using IConvertible does avoid the boxing, which is nice.

[lipchev]

LGTM
Hopefully, one day we would be able to add the properties here as well..

[angularsen]

Looks good to me also, thanks!

@lipchev you can also merge PRs by the way unless you want a pair of extra eyes on it.
I'll probably look over it anyways, but no need to wait for me.

[lipchev]

@angularsen I know, but I didn't know how you want to handle the release of this one, and the merging of the changes between master and release-v6..