feat: update rive package to 0.8.1 (now support raster graphics)

[nmarafo]

Description

Update rive package to 0.8.1 (now support raster graphics), in pubspec.yaml in flame_rive package.

Checklist

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• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples.

Breaking Change

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  e.g. feat!:, fix!:).
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Related Issues

Fixes #1270

[spydon]

Lgtm, nothing else that has to be done than just bumping the version?
@renancaraujo