fix: StacColorSchemeParser background and onBackground required

[deveminsahin]

Description
Although flutter deprecated background and onBackground for theming, they are required on the parser

Steps To Reproduce

1. Go to 'stac_color_scheme.dart' line 28,29

Expected Behavior

They should be nullable

Screenshots

Additional Context

Add any other context about the problem here.

[divyanshub024]

Hey @sahinemin!! Thanks for raising this issue. This issue has already been fixed here. Will send out a release with this patch soon :)

[divyanshub024]

Hey @sahinemin, here is a pre-release version with the patch https://pub.dev/packages/stac/versions/1.0.0-dev.2