Bump go_router from 14.8.1 to 15.0.0 in /platform_channels #6510

Job	Run time
web/samples_index config check	50s
ios-build	12m 48s
Test Flutter stable on ubuntu-latest	16m 17s
Test Flutter stable on windows-latest	27m 21s
Test Flutter beta on windows-latest	29m 30s
Test Flutter master on macos-latest	30m 55s
Test Flutter master on ubuntu-latest	27m 16s
Test Flutter beta on ubuntu-latest	17m 12s
Test Flutter beta on macos-latest	18m 33s
Test Flutter master on windows-latest	38m 28s
Test Flutter stable on macos-latest	15m 40s
	3h 54m 50s

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