Bump deployment target to 10.15 according to latest stable

[AbdeMohlbi]

this change Bumps the used MACOSX_DEPLOYMENT_TARGET to 10.15 according to latest stable.

when running the bin/generate.dart script this warnings does appear

Updating minimum macOS deployment target to 10.15.

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• I listed at least one issue that this PR fixes in the description above.
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[Piinks]

Thanks for fixing this!! It looks like the analyzer is a bit unhappy. How can I help?

[AbdeMohlbi]

Thanks for fixing this!! It looks like the analyzer is a bit unhappy. How can I help?

Hello this is a bit compilcated to explain but according to what stuart said we must wait until a new version is deployed, see https://discord.com/channels/608014603317936148/692831010445983785/1498996963507441755

[AbdeMohlbi]

requires flutter/flutter#185765