Update License

[notshi]

The font has been included in Adobe Fonts since 2019 under the SIL Open Font License (OFL) license and should be reflected in this repo and main website font download.

Expected Behavior

Update the current CC-BY-SA-4.0 License to SIL Open Font License, Version 1.1

This should be reflected in the following places:

Repo
README.md
LICENSE.txt

Releases
README.txt and LICENSE.txt of the font zip files in Assets

Possible Solution

Scroll to Open source details on Adobe for links and descriptions.

Happy to do a pull request for repo updates if needed.
Thank you for creating and open sourcing Gilbert!

Context

I am currently using GIlbert in publications for our local community radio station in the upcoming LGBTQ+ History Month and wanted to confirm the conflicting license information.