first stab at v3

[mgravell]

Initial changes:

• tfm net461 -> net462, so allow:
• use BCL async interfaces on netfx
• add net7 tfm to allow DbDataSource
• remove this on SqlMapper connection methods
• new DbConnectionExtensions
• new DbDataSourceExtensions
• add CommandFlags to all APIs
• add CancellationToken to all async APIs
• subst CommandType? = null to CommandType = default
• subst int? = null (timeout) to int? = {some default TBD}

Not yet started: all multi-map APIs

To investigate: does CommandTimeout make sense on async APIs?

[mgravell]

Xref #1976

[iamcarbon]

.NET 7.0 will be EOL on May 14, 2024. It may be worth skipping the .NET 7.0 target and requiring .NET8.0 for DbDataSource.

[mgravell]

@iamcarbon for now I'm happy to keep net7 - that also gives us the ability to use the regex codegen tool (adding that to "main" right now, no API change except a net7 tfm); if net7 adds cost, and/or we want to add net8 APIs: that becomes more pressing, but right now: meh