Update all our .NET executables to target .NET 7.0.x

[konrad-jamrozik]

This is an issue similar to this one:

• Some of our internal pipelines fail to build due to using out-of-support .NET SDKs #4934

but this time we are moving to .NET 7.0.x, not .NET 6.0.x.

This work was proposed as a result of my conversation with @weshaggard re this PR:

• Install .NET 6.0.x runtime. Remove DotNetCoreVersion param. Undo DOTNET_ROLL_FORWARD. #5395

And triggered by @ckairen's attempt to target .NET 7 in this PR:

• stress test docker baseimage updates #5328 (review)

@weshaggard and my general idea is that our executables target framework major version should never differ by more than 1. That is, if our global.json lowest supported .NET runtime is 6, the newest any of our tools can target is 7. If one wants to target .NET 8, one first needs to ensure all our projects target at least .NET 7. Once this is done, then one can start targeting .NET 8.

Thus, given we are about to have first project targeting .NET 7 due to #5395, I am filling an issue to make everything target 7.