Follow-up to https://github.com/flutter/flutter/pull/143347.

As https://github.com/flutter/flutter/pull/143403 uncovered, deprecation warnings were still enabled for the mega_gallery because it doesn't use the analysis_options.yaml file from the flutter root. This change injects a analysis_options.yaml file for the mega_gallery that disables those warnings.

Submitting this separately from https://github.com/flutter/flutter/pull/143403 in case it affects the mega_gallery benchmark to get a clean new baseline before removing all the `deprecated_member_use` ignores.