Update license to AGPL3

[tocsoft]

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

Just setting up for releasing the updated betas for fonts & drawing to match the RCs

[codecov]

Codecov Report

Merging #135 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #135   +/-   ##
=======================================
  Coverage   78.08%   78.08%           
=======================================
  Files          71       71           
  Lines        2377     2377           
  Branches      291      291           
=======================================
  Hits         1856     1856           
  Misses        453      453           
  Partials       68       68           

Flag	Coverage Δ
#unittests	78.08% <ø> (ø)

Impacted Files	Coverage Δ
src/SixLabors.Fonts/AppliedFontStyle.cs	75.00% <ø> (ø)
src/SixLabors.Fonts/BinaryReader.cs	64.86% <ø> (ø)
src/SixLabors.Fonts/Bounds.cs	100.00% <ø> (ø)
src/SixLabors.Fonts/Exceptions/FontException.cs	0.00% <ø> (ø)
...rs.Fonts/Exceptions/FontFamilyNotFoundException.cs	0.00% <ø> (ø)
src/SixLabors.Fonts/Exceptions/FontsThrowHelper.cs	0.00% <ø> (ø)
...ixLabors.Fonts/Exceptions/GlyphMissingException.cs	0.00% <ø> (ø)
...abors.Fonts/Exceptions/InvalidFontFileException.cs	100.00% <ø> (ø)
...bors.Fonts/Exceptions/InvalidFontTableException.cs	100.00% <ø> (ø)
...bors.Fonts/Exceptions/MissingFontTableException.cs	100.00% <ø> (ø)
... and 61 more

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