Add NetCoreAppCurrent configurations to Microsoft.Extensions libraries

[eerhardt]

This makes all Extensions projects consistent in which TFMs they target. This way we don't need to add new TFMs during development of a new feature.

Fix#54012

It is probably easiest to review the commits separately.

1. Change all Extensions .csprojs to support NetCoreAppCurrent and NetCoreAppMinimum TFMs. Also make the .csprojs use a consistent pattern.
2. Fix code warnings/errors now that the Extensions libraries are building for NetCoreApp.

@buyaa-n - can you review the supported platforms work I did in the 2nd commit? (note that I ran into dotnet/roslyn-analyzers#4282 and worked around it with redundant Debug.Asserts).

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

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Issue Details

---

This makes all Extensions projects consistent in which TFMs they target. This way we don't need to add new TFMs during development of a new feature.

Fix #54012

It is probably easiest to review the commits separately.

1. Change all Extensions .csprojs to support NetCoreAppCurrent and NetCoreAppMinimum TFMs. Also make the .csprojs use a consistent pattern.
2. Fix code warnings/errors now that the Extensions libraries are building for NetCoreApp.

@buyaa-n - can you review the supported platforms work I did in the 2nd commit? (note that I ran into dotnet/roslyn-analyzers#4282 and worked around it with redundant Debug.Asserts).

Author:	eerhardt
Assignees:	-
Labels:	area-Infrastructure-libraries, new-api-needs-documentation
Milestone:	-

[tarekgh]

?>>

why we removed the nullability here? Activity.Baggage is already defined as nullable I guess

runtime/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs

Line 323 in 69b5d67

publicIEnumerable<KeyValuePair<string,string?>>Baggage

[eerhardt]

Because the rest of the library doesn't have nullability enabled. And since we don't, the compiler is warning. These changes will likely be undone when we enable nullability in this library.

[tarekgh]

Added some questions, LGTM otherwise. I assume @ericstj will take a look too.

[buyaa-n]

@buyaa-n - can you review the supported platforms work I did in the 2nd commit? (note that I ran into dotnet/roslyn-analyzers#4282 and worked around it with redundant Debug.Asserts).

LGTM, thanks!