v3.0.0

• #131 - move to Microsoft.Extensions.Logging v2.0 dependencies throughout, drop 1.x .NET Core versions (@DigiBanks99)
• #132 - SerilogLoggerFactory and AddProvider() support (@nblumhardt)
• #138 - move to current SDK (@SimonCropp)
• #140 - remove unused build-time constants (@nblumhardt)
• #141 - make LevelConvert level-mapping routines public
• #148 - build with VS2019 (@nblumhardt)
• #142 - (partially) cache low-numbered event ids (@nblumhardt)