Standardize on new two-line license header

[am11]

Similar to dotnet/runtime#41783 for arcade.
cc @jkotas

[jkotas]

Thanks

[am11]

Script used for this on macOS bash:

$ git remote add dotnet https://github.com/dotnet/arcade

# delete entire line containing the unwanted license third line in C# code, which are part of the whole repo
$ git ls-files :/*.cs |\
    xargs grep -l "See the LICENSE file in the project root for more information." |\
    xargs sed -i '' '/See the LICENSE file in the project root for more information/d'

# delete part of line containing license third line in rest of the files, which are part of the whole repo
$ git ls-files |\
    xargs grep -l "See the LICENSE file in the project root for more information." |\
    xargs sed -i '' 's/ See the LICENSE file in the project root for more information.//g'

# middle grep is there to avoid autofixing of abnormal file-endings (and errors line `sed: RE error: illegal byte sequence`)

after that it can be verified that there are no files containing the unwanted third line:

$ git grep -l 'See the LICENSE file in the project root for more information'
# or without -l to see the file contents