Remove/update version-specific code paths

[sophiewigmore]

Background

In the process of removing .NET Core 2.1 from the buildpack, we stumbled across multiple explicit code paths that were in place for .NET Core 1.X and 2.1. We removed some of these (change 1 and change 2).

Issue

Since this wasn't our prime objective while removing 2.1, we only removed explicit version calls. We discovered less obvious code paths such as the one files in #447 that use out of date .NET Core features.

In this issue, we should do a buildpack-wide search of the different criteria we use for detection, supply, finalizing, etc and:

• identify out of date logic (for example, what we did in 69739bf)
• identify new logic we don't support for 3.1 and 5.0 (for example, searching for FrameworkReference in Look at FrameworkReferences in addition to PackageReferences #447)

Depending on the difficulty of the change, you may either fix it as a part of this issue, or file a separate issue to tackle it and link to that issue in the comment section of this issue.

It may be useful to reference the official .NET Core version migration guides for this:
https://docs.microsoft.com/en-us/aspnet/core/migration/22-to-30?view=aspnetcore-5.0&tabs=visual-studio
https://docs.microsoft.com/en-us/aspnet/core/migration/30-to-31?view=aspnetcore-5.0
https://docs.microsoft.com/en-us/aspnet/core/migration/31-to-50?view=aspnetcore-5.0

Issue blocked on #446

The completion of this issue will unblock this issue. It's completion will lead to 5.0-specific code paths being added which takes care of a part of the "update" step in this issue.

Acceptance

Acceptance for this issue is tricky since there isn't a nice definitive list of features to check for. For each identified piece of logic for removal or addition, it should be fixed as a part of this issue or a new issue should be filed.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/179784293

The labels on this github issue will be updated when the story is started.

[johnnyr0x]

Is this still applicable? @sophiewigmore

[johnnyr0x]

• Is it still applicable?
• If so
  • How important is it to do? What is the impact of not doing this?
  • How much engineering effort?

[johnnyr0x]

Per our discussion this 3rd bullet as well:

• What context would you share with someone who hasn't seen the issue?