Add watchOS BugsnagNetworkRequestPlugin scheme

[kstenerud]

Xcode sometimes lies about creating a public scheme for a target. You have to manually unselect "public" and then re-select it to generate the file :/

Also fixed up the watchOS versions for the network plugin targets. Minimum OS is 6.0 (where NSURLSessionTaskMetrics are introduced)

[github-actions]

Bugsnag.framework binary size decreased by 3,816 bytes from 819,568 to 815,752 🎉

    FILE SIZE        VM SIZE    
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  -0.5% -3.73Ki  -1.9% -16.0Ki    TOTAL

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