V4 Development: DefineConstants requires an overhaul

### Describe the feature

The constants defined in the project have grown over the years and are a bit messy. There is still BCL constants defined and some of the constants look like they are the standard constants but are actually not. It would be good to do a general sweep over the v4 development branch and get rid of unnecessary constants, BCL definitions and rely as much as possible on the constants already defined by the SDK.

Also some of the defined constants are unnecessarily specific. For example it would be enough with the current target frameworks to use NETSTANDARD, NETCOREAPP and NET and once NETCOREAPP is dropped just NETSTANDARD and NET.

### Use Case

Less cognitive overhead for everyone involved

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS .NET SDK and/or Package version used

All

### Targeted .NET Platform

All

### Operating System and version

All

Start by surveying the constants across the v4 development branch and comparing them with constants supplied by the .NET SDK. Identify unnecessary BCL definitions, misleading standard constants, and overly specific target-framework constants. Done means the unnecessary definitions are removed and the remaining constants use the SDK or the specified broader framework names.

Written by the indexing model from the issue text.