[google_fonts] Replace deprecated FontWeight.index (#10544)
Replaces usage of the deprecated FontWeight.index with value where a specific value is needed, and with direct comparison against a FontWeight constant when checking for normal font weight.
Fixes https://github.com/flutter/flutter/issues/179272
## Pre-Review Checklist
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Remove "All right reserved" from all files (#10066)
Per guidance from legal on current preferred practice, remove "All rights reserved" from all LICENSE files and header blocks, and update the repo tool check accordingly.