Drop support for .NET 6 and .NET 7 in Dapr v1.16

[WhitWaldo]

Describe the feature

Dapr v1.15 will feature support for .NET 6, .NET 7, .NET 8 and .NET 9 even though both .NET 6 and .NET 7 are both officially out of support right now.

With the release of Dapr v1.16, we'll be dropping support for .NET 6 and .NET 7 retaining support only for .NET 8 and .NET 9. With this shift, we'll also migrate the minimum language version to C# 12 from C# 10.

We intend to announce this in the v1.15 release notes as well, but as both .NET 6 and .NET 7 are officially out of support themselves at this point, we will seek to deprecate both of these versions in the next release of Dapr as well.

Release Note

RELEASE NOTE: UPDATE Remove .NET 6 and .NET 7 support from Dapr .NET SDK