3.0.0 Release

[nblumhardt]

• Upgraded Microsoft.Extensions.Logging NuGet packages to 2.0.0 #131 - move to Microsoft.Extensions.Logging v2.0 dependencies throughout, drop 1.x .NET Core versions (@DigiBanks99)
• SerilogLoggerFactory and AddProvider() infrastructure #132 - SerilogLoggerFactory and AddProvider() support (@nblumhardt)
• move to current sdk #138 - move to current SDK (@SimonCropp)
• Remove some unused build-time constants #140 - remove unused build-time constants (@nblumhardt)
• Make the level mapping routines public #141 - make LevelConvert level-mapping routines public
• Update to use VS 2019 final image for build #148 - build with VS2019 (@nblumhardt)
• Cache low-numbered numeric event ids to reduce allocations #142 - (partially) cache low-numbered event ids (@nblumhardt)

[nblumhardt]

Folks, any thoughts/feelings/perspectives we're missing on whether this should go out the door as-is? We've had the brakes on in case there were breaking changes in the ASP.NET Core 3.x series, but it doesn't seem like that's going to be the case, so we should probably get this all out in the wild :-)

[nblumhardt]

Ideally before merging this we'll finish #142 and #148

[adamchester]

LGTM

(So far...)

[nblumhardt]

#142 is ready to go