Upgrade Functions V1 Host from .NET 4.7.1 to 4.7.2

#### What problem would the feature you're requesting solve? Please describe.

Currently, while .NET 4.7.1 is _technically_ compatible with .NET Standard 2.0, using any dependencies that depend on .NET Standard 2.0 imports dozens of libraries from CoreFX to allow that compatibility with .NET Standard 2.0. This is obviously not ideal.

#### Describe the solution you'd like
Upgrade the target version of Functions V1 host from .NET 4.7.1 to 4.7.2. This will make it easier for both customer code and extensions to use .NET Standard 2.0 dependencies. It shouldn't be a breaking change either, as it is not a major version change.

Locate the Functions V1 host project and its target framework declaration, then inspect how the host and its dependencies build against .NET Standard 2.0. Verify the target change with the repository's existing build and test process; done means the V1 host targets .NET 4.7.2 without breaking compatibility.

Written by the indexing model from the issue text.