Update README.md to correct supported .NET (Core) versions

[lauxjpn]

Providers are usually compatible with future .NET versions, not just the version that was current at the time of the release.

(They are usually only compatible with one specific EF Core major version though.)

[ChrisJollyAU]

But EF Core only targets certain .Net versions, that why I had it like that.

EF Core 6 only targets the last LTS .Net version.

So if you were on .Net 6, you can use EF Core 6 but also upgrade to EF Core 7 while staying on .Net 6. However if you wanted to upgrade to EF Core 8, you would have to update to .Net 8

[lauxjpn]

While EF Core might target .NET 6, it is compatible with any .NET version from 6 onwards, as is EFCore.Jet. We do not need to provide the users with possible EF Core upgrade paths on our frontpage (or our repo at all, since this is an EF Core matter).

Users want to know, what technology stack they can use with which version of EFCore.Jet.

(Also, many users think that they have to use EF Core 8 if their app is targeting .NET 8, which is not the case. So a matrix like this one usually clears-up those misconceptions effectively.)

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BTW, on the Pomelo repo, we are deliberately keeping all relevant release trains in the compatibility matrix, especially the EF Core 3.1 compatible one, since this is the only official upgrade path for EF6 apps.