Update to .NET 6 and new License.

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

This PR touches a lot of files (I'm sorry) but was the absolute minimum I could do in order to upgrade the project and fix all the build warnings.

It's all cosmetic with zero functional changes. Basically

• Updates target frameworks and dependencies
• Updates shared infastructure
• Fixes new build warnings
• Updates license, readme, and ci builds
• Removes obsolete compiler conditions

[codecov]

Codecov Report

Merging #351 (3199290) into main (32bef42) will increase coverage by 0%.
The diff coverage is 82%.

@@          Coverage Diff          @@
##            main    #351   +/-   ##
=====================================
  Coverage     86%     86%           
=====================================
  Files        238     238           
  Lines      15187   15169   -18     
  Branches    2110    2099   -11     
=====================================
  Hits       13071   13071           
+ Misses      1624    1613   -11     
+ Partials     492     485    -7     

Flag	Coverage Δ
unittests	86% <82%> (+<1%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...ixLabors.Fonts/Exceptions/GlyphMissingException.cs	0% <0%> (ø)
src/SixLabors.Fonts/IGlyphRendererExtensions.cs	0% <0%> (ø)
...SixLabors.Fonts/Native/MacSystemFontsEnumerator.cs	0% <0%> (ø)
...nts/Tables/AdvancedTypographic/GPos/AnchorTable.cs	47% <ø> (ø)
....Fonts/Tables/AdvancedTypographic/GPos/AnchorXY.cs	100% <ø> (ø)
.../Tables/AdvancedTypographic/GPos/BaseArrayTable.cs	100% <ø> (ø)
...onts/Tables/AdvancedTypographic/GPos/BaseRecord.cs	100% <ø> (ø)
...ts/Tables/AdvancedTypographic/GPos/Class1Record.cs	100% <ø> (ø)
...ts/Tables/AdvancedTypographic/GPos/Class2Record.cs	100% <ø> (ø)
...Tables/AdvancedTypographic/GPos/ComponentRecord.cs	100% <ø> (ø)
... and 211 more

... and 3 files with indirect coverage changes

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[JimBobSquarePants]

@SixLabors/core I'm going to push this as it's non-functional and asking anyone to review nearly 400 files is too much .

[djrecipe]

Bummer that you guys are migrating to .NET 6. I imagine you must be consuming dependencies which are .NET 6 only?

A lot of us are going to be stuck supporting .NET standard 2.0 for a bit longer, so I guess we'll be stuck with fonts 1.0.1 for awhile 🙃.