Update to license enforcing submodule

[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 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

Updates the submodule to point at the version supporting new build requirements

See:

https://sixlabors.com/posts/licence-enforcement-changes/

[antonfirsov]

Thought on the blogpost language:

Starting with future major releases, a sixlabors.lic file will be required to build projects that directly depend on Six Labors libraries.

Instead of future major releases It might be worth to say 4.0+ or similar to exclude any ambiguity.

[JimBobSquarePants]

Yeah, I should specify the full list. I’ll update tomorrow

@antonfirsov Done