Lower down the minimum .NET Framework version to 4.7.2 if possible

[OPCLabs]

Type of issue

• Bug
• Enhancement
• Compliance
• Question
• Help wanted

Current Behavior

Version 1.5. appears to require .NET Framework 4.8.

Expected Behavior

As far as I know, the codebase can work with .NET Framework 4.7.2 without change too, and my customers may require it.

Steps To Reproduce

1. Open https://www.nuget.org/packages/OPCFoundation.NetStandard.Opc.Ua.Client/1.5.374.26-preview .
2. .NET Framework 4.8. is listed; .NET Framework 4.7.2 is not listed.

Environment

- OS: any
- Environment: any
- Runtime: .NET Framework 4.7.2
- Nuget Version: 
- Component:
- Server: yes
- Client: yes

Anything else?

Discussed in person with @mregen.