Release Rx.NET v6.0.0-preview.1 · dotnet/reactive

First build with net6.0 targets, and with test support for `net7.0.

What's Changed

Remove support for .NET Core 2.1 and netstandard1.0. bump .net to 4.8 by @clairernovotny in #1661
Bump MSBuild.Sdk.Extras from 3.0.38 to 3.0.44 in /Rx.NET/Source by @dependabot in #1669
Bump MSBuild.Sdk.Extras from 3.0.38 to 3.0.44 in /Ix.NET/Source by @dependabot in #1670
Bump Verify.Xunit from 14.7.0 to 14.11.2 in /Rx.NET/Source by @dependabot in #1668
Bump IsExternalInit from 1.0.1 to 1.0.2 in /Ix.NET/Source by @dependabot in #1666
Fix defines to be OR_GREATER by @clairernovotny in #1671
Bump coverlet.collector from 3.1.0 to 3.1.1 in /Rx.NET/Source by @dependabot in #1696
Bump coverlet.collector from 3.1.0 to 3.1.1 in /Ix.NET/Source by @dependabot in #1694
Bump FluentAssertions from 6.2.0 to 6.4.0 in /Ix.NET/Source by @dependabot in #1688
Update Merge.cs by @theodorzoulias in #1684
Bump Nerdbank.GitVersioning from 3.4.244 to 3.4.255 in /Rx.NET/Source by @dependabot in #1680
Bump Nerdbank.GitVersioning from 3.4.244 to 3.4.255 in /Ix.NET/Source by @dependabot in #1681
Add MinByWithTies and MaxByWithTies to bring back a MinBy/MaxBy that returns a list by @clairernovotny in #1700
Include icon in package & fix a couple warnings by @clairernovotny in #1701
Change master to main into PR template by @sungam3r in #1731
Update Observable.Single.cs by @theodorzoulias in #1717
Upgrade to supported TFMs and SDK versions by @idg10 in #1882
Add Rx.NET 2023 roadmap, and tooling update ADR by @idg10 in #1879
Set Rx vnext to 6.0 and add versioning ADR by @idg10 in #1886

Full Changelog: ixnet-v5.1.0...rxnet-v6.0.0-preview.1