change: Update project target frameworks and dependencies

[DenSmoke]

• Project target frameworks replaced with .NET Framework 4.6.1 and .NET Standard 2.0 to be consistent with the frameworks supported by other .NET libraries associated with Google Cloud Platform.
• Removed Google.Api.Gax
• Updated Google.Api.Gax.Rest to 3.2.0
• Updated Google.Apis.Auth to 1.49.0
• Updated System.Collections.Immutable to 1.7.1
• Fixed tests

API CHANGE: Dropped support for netstandard1.5 and net45 target frameworks. Developers are now required to use netstandard2.0 or net461.
API CHANGE: Upgraded the dependency Google.Api.Gax to the latest major version. Developers that use the PagedAsyncEnumerable interface from this package (usually as the return value of list iteration API calls), may have to update their implementations accordingly.

Resolves #264 #265 #207

[DenSmoke]

@hiranya911 @lahirumaramba Can someone review please?

[hiranya911]

@DenSmoke Thanks for the PR. Really appreciate your effort. This contains several breaking changes. Specifically, following changes are breaking:

1. Dropping support for netstandard1.5
2. Upgrading from Gax 2.0 to 3.0

We can look into accepting those changes in the next quarter, and releasing v2 of the SDK. Can I ask you to extract the above 2 changes from here into a separate PR? The remaining changes in this PR (e.g. upgrading test dependencies) can be merged immediately.

[DenSmoke]

@hiranya911 Upgrading test dependencies is not critical. Main purpose is to solve conflicts like in issue #207 when using GAX 3.0 dependent Google Cloud .NET libraries and Firebase Admin SDK in same .NET project

[hiranya911]

Yeah, I understand. We can hold on to this PR for now, and come up with a plan to merge it in the coming months. I just need to sync up with other teams and see if we have any upcoming releases that might potentially conflict with this.

[hiranya911]

We have slated this for a release in the next quarter (q4 2020). As a first step we will try to add net461 to the list of target frameworks without any additional changes, and release that. Then we can merge this PR on top of that.

[hiranya911]

@DenSmoke can I ask you to update this PR and sync up with the latest state of the master branch? We can merge this soon after that.

[DenSmoke]

@hiranya911 done

[hiranya911]

Thank you @DenSmoke. This LGTM. I'll merge it in a couple of days, and start working on a release.